Yes I would be concerned. What size tank is it in? Are there macro algaes growing in the tank? Tangs are very active fish and have a high metabolism rate. Hence they need to eat more, almost constantly.
That, and I would be surpised if it ate the brine. These fish are herbivores, they dont eat meat. Try feeding formula two frozen food instead. it is a seaweed based food for herbivores. That and Nori seaweed sheets.
I am no expert on tangs, but this is what I have been told . . . . which is what kept me from getting one.
Justin