That's most likely marine ich. When my fish had that I spent literally hours and hours reading about it. If you want to rid your fish of it you will have to set up a separate tank to treat them. If you don't want to do that you can feed them well, and giving them food soaked in garlic or Selcon will help somewhat and they might pull through it. It might keep coming back, and you may lose one or more of your fish. Read up on it. Just don't trust everything you hear, because there are a lot of rumors out there. A true cure takes weeks and you must treat in a separate tank. That's why many people just let their fish live with it (if it doesn't kill them). Good luck.