Im not sure what its called but I've noticed some dwarf angels and tangs doing this at my LFS, it sees to happen when they are stressed. How long has it been happening for, how big is the tank? how long are your lights on for?
Did you QT him? FW dip? At a minimum he has severe HLLE and possibly a bacterial infection starting. I see a spot in his tail (2nd pic) and possibly a spot in his dorsal fin (first pic). I would FW dip to check for flukes. You’re probably going to need a QT tank to treat with antibiotics.
I can't speak to the HLLE but my yellow tang recently had red blotches like that. I treated it with a combo of Metroplex and Kanaplex and she came through ok. Nitrofuracin green was recommenced but I didn't have any on hand.