It looks like a clump of fungus but could also be excess mucous.
How long has the tank been set up for?
Does the tank have a filter and if so, is it run continuously?
How often do you clean the filter and how do you clean it?
Have you checked the water quality for ammonia, nitrite, nitrate & pH?
If yes, what were the results?
How often do you do water changes and how much water do you change?
Do you dechlorinate the new water before it goes in the tank?
What other fish are in the tank?
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The first thing to do if you ever see a sick, dead or unhappy fish, is a 75% water change and complete gravel clean each day until the problem is identified or fixed. This dilutes nutrients and removes harmful pathogens that might be in the water.
Make sure you use buckets that are for the fish only and have never been used for cleaning or anything else.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
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If you only have platies in the tank you can add 2 heaped tablespoons of rock salt, sea salt, swimming pool salt or aquarium salt. If there is no improvement after 48 hours then add another 2 heaped tablespoons of salt so there is 4 heaped tablespoons of salt per 20 litres of water.
Keep the salt level like this for at least 2 weeks but no more than 4 weeks.
If you have other fishes in the tank, only add 1 heaped tablespoon of salt per 20 litres and if no improvement after 48 hours add another heaped tablespoon so there is 2 heaped tablespoons of salt per 20 litres.
To work out the volume of water in the tank:
measure length x width x height in cm.
divide by 1000.
= volume in litres.
When you measure the height, measure from the top of the substrate to the top of the water level.
There is a calculator/ converter in the "How To Tips" at the top of this page that will let you convert litres to gallons if you need to.