Funny mine has the same but his lip & on its side fin! I got him about 3+ wks ago, he looked fine, then the white stuff appeared in a couple days! After a wk it looked like it was gone then back worse! I talked to the shop where I got him & he said it's cuz it's young & it's immune system needs to mature & fight it off. He eats, swims & is active so far! I have a large black clown that sleeps & swims w/the new lil guy & she still is clean so I'm hoping he's right as I'd hate my other fish to all get polluted! I do wkly 20%water changes since I have a crowded tank & elevated nitrates but everything else is good!! He also said a cleaner shrimp might "fix" him too!