White "stuff" on clownfish mouth.

[Rondelet]

Don't forget that any fish, like the damsels, or inverts, such as LPS, can act as carriers - bringing pathogens into your system without showing signs of disease. Now that I've thought about it, it's possible that the clown ran into something or was in a fight and the white stuff is simply excess mucous and part of a skin healing episode. Whatever the reason, the good news is that it's going away!