How long has the tank been set up for?
What other fish are in the tank (it looks like you have 1 or 2 red parrot cichlids)?
How often do you do water changes and how much do you change?
Do you gravel clean the substrate when you do a water change?
Do you dechlorinate the new water before adding it to the tank?
Does the water have a milky cloudy appearance to it?
What are the ammonia, nitrite and nitrate levels in the water?
What sort of filter is on the tank?
How often and how do you clean the filter?
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It's not white spot. Ichthyphothirius (Ich or white spot) looks like grains of salt sprinkled over the fish.
The fish is covered in excess mucous, which is caused by something in the water irritating the fish. In response the fish increases the amount of mucous it normally produces and develops a cream or white film over the body.
Excess mucous is normally caused by poor water quality (ammonia, nitrite or nitrate) but can also be caused by external protozoan parasites, chemicals (fertiliser, medication) in the water and chemicals getting into the water from outside the aquarium (smoke, paint, fumes, perfume, deodorant, hand sanitiser residue on your skin, etc).
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The white mouth is a concern and could be excess mucous or the start of mouth fungus (Columnaris). It it's Columnaris it will need antibiotics. However, before treating with anything, do the following.
Test the water quality for ammonia, nitrite, nitrate and pH. Post the results in numbers here.
Wipe the inside of the glass down with a clean fish sponge. This removes the biofilm on the glass and the biofilm will contain lots of harmful bacteria, fungus, protozoans and various other microscopic life forms.
Do a 75% water change and gravel clean the substrate every day for a week or until the problem is identified. The water changes and gravel cleaning will reduce the number of disease organisms in the water and provide a cleaner environment for the fish to recover in. It also removes a lot of the gunk and this means any medication can work on treating the fish instead of being wasted killing the pathogens in the gunk.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash the filter materials/ media in a bucket of tank water and re-use the media. Tip the bucket of dirty water on the garden/ lawn. Cleaning the filter means less gunk and cleaner water with fewer pathogens so any medication (if needed) will work more effectively on the fish.
Increase surface turbulence/ aeration to maximise the dissolved oxygen in the water.
Post more pictures in 24 and 48 hours time so I can check the mouth. If it is Columnaris, it usually spreads rapidly over the mouth and face and the fish normally dies in a couple of days.