Hello I do not know what this is on my tang but I don't think it has been there help!!!!
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hoping for the best, sometimes their coloring can get a little spotchy. Could be due to stress, make sure your water quality is spot on and enrich his food. I don't feed nori, but you could; I inturn feed any fish I feel compromised with many small meals throughout the day. With tangs I feed hikari mega algae and new life mega-algae.
I see the ring you're talking about, below his gill and pretty close to where his heart should be, but I don't immediately recognize it. Perhaps @melypr1985 or other members of the #reefsquad have seen something like that before . . .
~Bruce
Also, acanthurus tangs (especially powder blue, brown, gold rim, and Achilles) will lose color and discolor when ich or velvet infest their gills (often before you see anything on the fish). Watch closely. If you didn't treat in copper or CP prior, I would be HIGHLY suspicious and be ready to properly QT with these guys.
Unfortunately, I think this will not be the last you see of ich... be diligent!When I first got him he had ick and I tested it with this treatment called hyper that always worked good and it looks like the spot is going away... fingers crossed
Unfortunately, I think this will not be the last you see of ich... be diligent!

