My yellow tang has a couple white sponge like things on him. One is on his left fin and the other is on top. These have been there for a few weeks already and don't seem to be spreading or bothering him at all. He eats very aggressively and the one on top had actually gone away but came back a couple days later. It does not look like ich and has not affected any of my other fish. Any ideas on what it is and what I can do to treat it?