Appears to be Lymphocystis which if often associated with Poor water quality or poor diet or both.
The water can be from your LFS and not necessarily yours but do test and monitor your values(parameters). If youre Using API test kits, you could be getting false test results.
If you are using API, take a good water sample to a trusted pet store that does NOT use API kits and have them test for you to see what results they get and to compare with yours.
For diet, Feed:
Formula flake and frozen
LRS Fish Frenzy
mysis shrimp
spirulina brine shrimp
Hikari Marine cuisine
brine shrimp flakes
Add Garlic extract to foods occasionally for immunity health. Assuming it is lympho, with proper water conditions and diet, it will fall off by itself
