You didn't post enough information. For instance, if this was a recently acquired fish, the condition might be from a net, capture, or other transportation injury.
If the fish is an 'old' pet, it may have had a run-in with a piece of rock, decoration, or coral. Tangs are not the 'sharpest knife' in the drawer and in their all-too-often mad dashes, they run into things. It looks like this guy forgot to duck.
Another possibility is the onset of a bacterial infection brought on by either a chronic or acute stress source or sources. In this case, if it gets worse or spreads, move the fish into a hospital tank and give it a treatment using an antibiotic.
In all of the above scenarios, the fish is irritated. Flashing or 'scratching' might be a normal response to this condition.