Probably lymphocystis. It's a harmless virus. Only time it is a problem is if it impairs the fish's ability to eat or breathe. Best treatment is pristine water quality, a wide variety of foods with vitamins. Especially adding Beta Glucan to the food mix. It really helps the fish's immune system to suppress the virus. Here's an article from the Advanced Aquarist on Beta Glucan. People take it too for the immune system boost.
Wouldn’t it be great if there were one thing that we could give any sick fish that could help it? Aquarists dream of a “magic bullet” that could be used as a treatment for a plethora of problems.