it's called ich or ick. sorry i don't know much about it i just happened to be looking through a marine begginers guide and that was a common disease/ infection. keep in mind i don't know much of it besides that it's called ich. "Most wild fish are exposed to low levels of this parasite but are able to effectively fight off the infection without becoming seriously ill. In contrast, due to a relatively small volume of water and a concentrated population of fish, the number of Cryptocaryon has the opportunity to explode in the home aquarium."(liveaquaria.com). just look it up and find out how to resolve this situation.