try some dried seaweed from the lfs
or if you live in a locatoin that has a decent asian market or section of your grocery store get some dried nori.
I feed my PBT an almost exclusive diet of dried nori.
Tangs over all are grazers. They should have food in the tank available for them to eat thru out the day or atleast for an extended period of time like several hours.
I am supprised it did not go after the brine shrimp as mine will do this aswell but it should not be the bulk of its diet.
What other fish are in the tank. How big is the tank? How big is the PBT?
It could be that the tang is stressed to a point it does not want to eat what is being offered.