Emperor angels develop those light spots whenever they have external parasites. Could be from ich, but also could be from flukes. The way I tell the difference, is to see if there are any discrete white spots on the fish's fins - then it is ich. If the light spots are only on the body, then it is flukes. Trouble is, this picture isn't clear enough to see the fins.
Without a positive diagnosis, a 5 minute FW dip followed by a copper treatment in a quarantine tank would be the way to go. Or, hyposalinity - that will handle ich and most flukes.