It looks lik he might have head and lateral line erosion. (I'm looking at a white patch behind his eye, and it looks like some of the fins have receded.) It seems to be fairly common in tangs and large angels.
I'm not sure it's curable, and I don't think they really know what causes it. The normal prescription I've seen referenced calls for water changes to minimize nitrates along with variety in the diet. I've also read that some have had success wiht medicated food under the supposition it is casued by a parasite. Metro+ is one food that can be mixed with frozen food.
I don't think it's immediately fatal, but the fish often looks very very rough as the problem continues to progress.
It is a violation of federal law to use this product in a manner inconsistent with its labelling.