I doubt that it would introduce it into your tank. You've already got it. But because your tangs constantly graze you can't see it. Looks like they're nibbling at bare rock?

Also, heavily algaed rocks are often ignored by tangs. I have taken to putting rocks into the tang tank to clean it up, but the algae starts growing immediately.

To echo the others, get nori. If you're aiming to shave a few dimes, get it from the Asian market.