Cremesicle Mollies Looked "dirty", Now Are Losing Color?

I have a 55 gallon brackish tank that currently has 4 adult mollies (1M:3F), a dragon goby, 3 baby columbian shark catfish and a bunch of molly fry in breeder nets. About two days ago I was looking at them and I noticed one of my cremesicle mollies had what looked like dust or very fine dirt on her, not white like ich but a dark brownish color. I looked up "dark specks on fish" and I never saw any pictures of what my fish looked like and since the fish was acting completely normally I didn't worry about it too much at the time. Yesterday I noticed that there seemed to be some on my other cremesicle molly as well, and the edges of her fins had a darker kind of dirty look to them, but both were still perfectly active and eating well. I woke up today and checked once more and now there are no specks and the dirty looking fin edges have disappeared but both cremesicle mollies are losing the orange color on their bodies, are less active and have a strange shiny or pearly scale look on the very tops of their bodies. I have not noticed any flicking. They have been eating well throughout this and still are. At first I thought it could have been velvet but looking at pictures, this looked nothing like it yesterday and today it's as if they have a different problem completely.. I am really not sure what could be wrong with them, does anyone have an idea? I am limited on fish medications at the moment,   what I have on hand is Pimafix, Ich Quick-Cure and Parasite Guard. I know I have Tetracycline as well somewhere but I can't find it at the moment. I can buy something else if anyone has a good guess but I can't afford to buy a lot of different medications if i'm not sure they are what I need.
 
Info about the water.. ***I use the Freshwater Master kit from API so i'm not sure if anything would be off..?
 
1.008 s/g
 PH: 7.8* 
Ammonia: 0
Nitrite: 0
Nitrate: 50 ppm
 
I see that my Nitrates are high so I will definitely be doing a water change today, but that doesn't explain the problem with my mollies and why it is only affecting my cremesicle ones. 
 
Do you think I should quarantine these two mollies? Whatever the problem is, it seems to be completely isolated to my orange ones, but I have molly fry as well and since I don't know what this is, I should probably prevent the babies from being exposed to it any longer. 
 
I apologize for the bad picture, they were acting sluggish up until I got my camera- which of course is when they decided it was time to freak out and swim around so I couldn't get a good one. I hope it is clear enough. To get an idea of how much they have faded, the bright orange on that one's tail is the color that their entire bodies were up until yesterday.
 
Any help would be appreciated, thank you.

Do the brown specks on the fish look like velvet patches, or brown fluffy specks.
It could anything from velvet to columnaris.
If it was velvet would expect to see some signs of flicking and rubbing.

I was going to say velvet also known as gold dust disease...
http/mercurio.hubpages.com/hub/velvet-disease-rust-disease-gold-dust-disease

The thing is that the specks are completely gone today. They didn't look fluffy or anything like that, they looked like tiny little specks smaller than a grain of table salt but they were brown in color, and they were scattered around just kind of sitting on it as if it had been rolled in dirt. Today there are no specks, the color in both my cremesicles is disappearing though. Looking at the pictures, I still say velvet doesn't look familiar at all with what has been going on the past couple days. There is no change to the texture of the fish's scales or skin, no bumps or patches at all. I had a columnaris problem in a different tank a while back and this doesn't look anything like that at all, either.

Keep a look out as it could be velvet.
They can look dirty in colour as well.
Then there columnaris which will look like brown patches, or brown fluffy patches.
 
Get you some information on velvet.

http/www.skepticalaquarist.com/oodinium

Could they still be maturing?
When did you get them?
What is your set up like?

I have heard that fish change colour to show their dominance and less dominant fish blend into their surroundings.

If it is an age thing then maturing fish change colour as they grow...

But I would add marine salt providing other tank inhabitants are OK with salt and raise temp a couple of degrees for 2 weeks with water changes every two days paying attention to the bottom of the tank cleaning the floor thoroughly.

If it gets worse then add meds. Esha exit and esha 2000 can be used together safely and effectively.

If no change and they are OK then I would just put it down to one of the above.

I thought velvet could show it self in really tiny spots the colour of a light brown, not dark brown.
I would just wait and see for now.
 
Good Luck.
 
http/www.angelfire.com/blues/fish_problems/VELVET.html

Nic1, I keep my tank almost at 80 degrees, which is what my heater maintains if I try to adjust the temperature. It either lets the water be 73 or 80 degrees because my last heater I was using I have learned has an exploding problem so I stopped using it. I can't make the water warmer but I have heard of moliles becoming lethargic at higher temperatures anyways so I would not want to do that. My water is also already brackish if you read the information about the water, I have been slowly raising it for a while waiting for my sharks to grow so I can transfer them out.
 
I got them a few months ago, they were young adults when I bought them.
What do you mean by "my set up" ?

I was suggesting you raise the temp for a couple of weeks, but if you do this you need to add an air stone to promote more oxygen..
Set up, I meant stuff like substrate, heavily planted, bog wood ect, was asking because of the theory of the Molly's adapting their appearance to their surroundings according to dominance.

I'm still thinking this may be bacterial though, its so odd, maybe a slime coat bacteria...
I'm going to do some reading for you will let you know what I come up with.

All good info about the salt, temp sect gives me a place to start.

.... OK so here is what I have researched.
It is not that uncommon, plenty of people have experienced their Molly's fading, even black ones and 90% of the them have gone on to live normal lives disease free.
I personally have kept Molly's for years and years and years, and have never come across this.
I have always been a firm believer in water being at its up most and this should maintain healthy stock, rarely having to use meds only in some circumstances when a disease won't budge!
The reasons I've listed above are commonly what have been used as a diagnosis, and Molly's blending into the colour of the substrate was used the most.
Also fish adapting to new set ups, being that you are converting to brackish and they were young adults could also be a cause for this.
I think that if no other symptoms are present then you can put it down to 'just one of those things!'
Sorry I couldn't give you a direct answer, this seems to of baffeled fish keepers for a very long time!