It looks a little like Hyper-Melaninization. Just to be safe/sure, is the fish around any coral or other invertebrate that might be stinging the fish? If you are sure the fish isn't likely being stung or irritated by an invert, then I would suspect a parasite/pathogen.
The photos are not close up enough for me to be sure however. Hate to ask, but. . .can you get photos that are up close to a few of those spots?
In any case -- being stung or a parasite/pathogen -- move the fish to a quarantine tank. If the matter is because of something in the DT then it should start going away. If it is a parasite/pathogen, then it will be in quarantine for the treatment.
