It would help if you can share your parameters & tank mates. Without knowing the numbers, looks like it could have nicked itself against the rock. My experience with those tangs is that they are very skittish & will dart behind the rock work when anything approaches the tank. If it can’t find an opening within the rock structure, it will jam itself between rocks in order to feel secure & that will cause injuries. It’s best to provide it with large openings in the rock structure where it can dart to & feel secure. Plus the fish looks pretty large & from your previous posts where you shared shots of your tank, the fish will have a hard time finding an opening fast enough that it can fit through with ease. I suggest building a large vertical opening towards the back of the tank that will make it easier for it to access when it needs to find some security. Another thing would be to introduce some sea weed rubber banded to a rock, if you haven’t already, near that opening in order to entice it to swim out in the open. My 2cents. Hopefully it helps & good luck.