ICH.....It is a parasite that your fish can get.
When your fish have it, they look like they have lil' white dots on them ( like you sprinkled salt on them)
Can be fatal to your fish and you need to treat them in your QT tank.
Do not treat them in your display tank if you have corals and inverts or if you plan on getting any. (medication has copper in it and it will kill corals and inverts.)