That is what I'd be leaning toward. Sometimes they can have a bad "reaction" for whatever reason (etiology unknown) and will "spot up" like that. I have seen them look terrible but do fine that way, too. Black ich, on the other hand, is very distinctive, raised, rounded black dots--not what your tang has. Then again--is it possible to get a clearer picture?
Well he died Monday. I am Super sad and really wondering what the deal was. I put him in the freezer because I dont know what else to do with him. It didn't feel right just putting him in the trash. Besides costing a butt load, I was attached. Weird I know. But I am super bummed.