I think the LFS is right. (!!!!).I've seen GTs with similar spots before, can't remember where.It is not HLLE, ich, etc. Lots of fish, tangs may be the most prone.
If nori is all the store-bought food; he's eating. Add some meaty stuff too. Mysis, a good pellet, gut loaded BS, Formula I, etc. Yes, these are herbivores, but wild tangs eat a lot of tiny crustaceans, pods, etc. I keep a lot of tangs and all are fed some type of meaty frozen food every day. Its not going to solve the spots issue, which I suspect will disappear with time, but the varied diet will help long-term health long term health. FWIW: It may be camera angle, lighting, etc., but this tang just doesn't look particularly to me. Are his fins always folded like this? Is he usually in this nose angled down position? He looks very lethargic, true?
To the OP: PM me, I'll send a link to an identical pattern on a huge GT. I can't post it here; too much junk from another forum, one that doesn't seem to allow members over the age of twelve.