That looks like ick to me. And once you see it you have to work quick to get rid of it. If it gets in their gills then they are toast. Ick works in cycles so you may see the spots one day and then not the next. Then all of a sudden all your fish have it. Once it gets in your system you need to either treat your whole tank (but not if it's reef). The best way is put the fish in QT and treat with copper or malachite green(Quick Ick).