The molly has excess mucous on its body.
The guppy looks like it has lost a couple of scales. It also appears to have gill flukes, although that is a minor issue, which can be treated with Praziquantel after we work out what is going on.
How long has the tank been set up for?
What is the ammonia, nitrite, nitrate, pH and GH of the tank water? Results in numbers please.
The GH (general hardness) and pH of your water can usually be obtained from your water supply company's website or by telephoning them. If they can't help you, take a glass full of tap water to the local pet shop and get them to test it for you. Write the results down (in numbers) when they do the tests. And ask them what the results are in (eg: ppm, dGH, or something else).
How often do you do water changes and how much water do you change?
Do you gravel clean the substrate when you do a water change?
What sort of filter do you have?
Is it run continuously?
How long has the filter been set up for?
How often do you clean the filter and how do you clean it?
What other fish are in the tank?
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Fish produce excess mucous due to poor water quality or chemicals and other irritants (protozoan infections) in the water. The best thing to do is a 75% water change and gravel clean the substrate every day for a week.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
If the filter has been running for more than 2 months, then clean it if it hasn't been done in the last 2 weeks. If you have a power filter, wash filter media in a bucket of tank water.
Do not replace filter materials unless they start to fall apart.
It looks like there is a lot of gunk on the bottom of the tank in the picture with the molly. Make sure you get this gunk out. Use a basic model gravel cleaner like the one in the following link to clean the gravel.
https://www.about-goldfish.com/aquarium-cleaning.html
The following link has information about what to do if your fish gets sick. It's long and boring but worth knowing. I recommend printing it out and reading it in bed to help fall asleep.
http://www.fishforums.net/threads/what-to-do-if-your-fish-gets-sick.450268/#post-3804819