Hey. Besides the things mentioned above. What’s your feeding regiment? Do you feed seaweed? Any new introductions to the DT? Any scratching against rock or coral? Have you noticed any change in behavior? My experience with those tangs has been a good one & they tend to be a strong active fish with a huge appetite, it’s the reason why I ask about diet & behavior. I am curious if you are housing any coral like an anemone (actually an invert) that the tang might have come in close contact with & stung itself with or aptasia, since tangs are grazers & can accidentally come in contact with stinging coral. But it does look like an external infection, i would add a cleaner wrasse to help clear & clean that infection or scarlet shrimp. The only other thing I can think of, is if your housing a rabbit or butterfly fish that might have extended its dorsal fin stingers while feeding & stung the tang. These are all scenarios I have experienced with my fish. One day they look fine & the next, they wake up with a gash because it fought over a piece of food with another mate or came close to a stinging coral. Just a few things to consider. Good luck.