Hi and welcome to the forum
It's either fungus or excess mucous, most likely fungus caused by rough handling.
The best treatment is salt.
You can add rock salt (often sold as aquarium salt), sea salt or swimming pool salt to the aquarium at the dose rate of 1 heaped tablespoon per 20 litres of water. If there is no improvement after 48 hours you can double that dose rate so there is 2 heaped tablespoons of salt per 20 litres.
If you only have livebearers (guppies, platies, swordtails, mollies), goldfish or rainbowfish in the tank you can double that dose rate, so you would add 2 heaped tablespoons per 20 litres and if there is no improvement after 48 hours, then increase it so there is a total of 4 heaped tablespoons of salt per 20 litres.
Keep the salt level like this for at least 2 weeks but no longer than 4 weeks otherwise kidney damage can occur. Kidney damage is more likely to occur in fish from soft water (tetras, Corydoras, angelfish, gouramis, loaches) that are exposed to high levels of salt for an extended period of time, and is not an issue with livebearers, rainbowfish or other salt tolerant species.
The salt will not affect the beneficial filter bacteria but the higher dose rate will affect some plants. The lower dose rate will not affect plants.
After you use salt and the fish have recovered, you do a 10% water change each day for a week. Then do a 20% water change each day for a week. Then you can do bigger water changes after that.
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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 it.
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Do not add any more fish to the tank until it has cycled in a couple of months time.
If the tank is only a couple of days old you will be doing a fish in cycle. This is where you have fish in the aquarium while the filters cycle (develop good bacteria to keep the water clean).
Anything that breaks down in water (fish food, fish waste, dead plant, dead fish) produce ammonia. In an established tank with an active biological filter, you get beneficial bacteria that eat the ammonia and convert it into nitrite. You get more bacteria that eat nitrite and convert it into nitrate. You get rid of nitrates by doing regular water changes (about 75% each week). Without an established biological filter, the ammonia levels build up and poison the fish.
It takes about 4-6 weeks for a filter to become established with the good bacteria. Until that happens you should keep the feeding down to a couple of times per week and do a 75% water change and gravel clean the substrate 4-8 hours after feeding. You should also monitor the ammonia and nitrite levels and do a 75% water change any time you have a reading.
Make sure any new water is free of chlorine/ chloramine before it is added tot he tank.
You can buy test kits from the pet shop to test for ammonia, nitrite, nitrate, pH and GH. There are "Master Test Kits" that contain all these tests or you can buy them individually. If money is tight then take a glass of tank water to the local pet shop and ask them to test the water for you. Write the results down in numbers when they do the test.
If you do buy test kits, try to get liquid test kits rather than dry paper strip kits. The liquid ones are slightly more accurate. Check the expiry date on them and don't buy kits that are kept near a heat source or in front of a window.
Keep test kits in a cool dry place away from light to maximise their shelf life.
If you want to help speed up the cycling process you can buy liquid bacterial supplements and add them to the tank. I recommend using a double dose every day for the first week, then add the rest. Try to add it to the aquarium near the filter intake so it gets drawn into the filter where it's needed.
If you post a picture of the filter and the name of it, we can tell you how to maintain the filter.
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What is the GH (general hardness) and pH of your water supply. This information 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).
Mollies need a GH around 250ppm and a pH above 7.0. If your GH is below 250ppm you will probably need to add some mineral salts to raise it. But find out what it is first.
