I am thinking it is stress. It could be brought on by several things tank mates, shipping, acclimation, food supply, different in the parameters all the different water it has been in since it has been caught. Your parameters might be perfect but what was the stores. Tangs are a very beautiful fish but at the same time so delicate.
Now I know several people that have tangs swear by garlic guard there is a marine biologist that had a powder brown tang that said tangs are seldom very happy in our tanks and should be left in the ocean. When he had to move to a new job he had to sell his tang he actually interviewed buyers for it. I listened to him give special instructions to the buyer so not to stress out the fish when putting in his tank. Give you an idea how picky he was he even took and tested the guys water for him to make sure it was good enough for the tang. He asked for anyone wanting the tang to send tank history and pictures. He said to give it garlic guard at least once a day soaked in seaweed. He went on to explain not all tangs will eat it but his did. I wonder if the guy has to send weekly pictures to prove how well the tang is doing lol.