In looking over your images - the small white powder-like dots aren't there in real life, right? The only issues are the big white spot on its left pectoral(side) fin and the caudal(tail) fin?
My first thought was Lymphocystis. While it could be that, the spots look too bright white and smooth to me. Lymphocystis is usually grayish and lumpy. You may know about this - if it is Lymphocystis, it is a viral disease and there is no treatment, but the fish will get better in 8 weeks or so.
Some background info would be helpful:
How long have you had the tang?
Did the spots show up all at once, or did they grow gradually?
Updated!!
I woke up today and feed her and realized that the little parasite is gone. I could not be more happy tanks every one.
The specks you see are particles of her food.
