It sounds like ich.
I leave any fish with ich in my display. Im sure ppl will say im a fool for doing this but, removing the fish to a qt tank will only stress them out further(ie, I had a hippo tang in my display for 6 months, I moved it to a qt b/c I thought it was eating my acans. However the move to the qt made the tang, which was healthy, break out in ich, and when I moved it back to my display, the ich came back b/c the fish was stressed out from being moved). Not to mention, that digging around your tank with a net will stress out all the fish/verts/corals, and you might break something, etc.
All fish healthy or not can catch ich, the healthy ones will fight it. Also, having some type of cleaner shrimp in a tank will really help, as the fish will go to the shrimp and get a good cleaning.
Also, as horrible as it sounds, if there is a weak fish in the system, the ich will kill this fish, thus removing any weak fish