that sounds like trematodes. They are a large fluke parasite. Often freshwater dips will solve the problem but it might take a few dips over several weeks. It is not common for yellow tangs to carry them. Angels, butterflys, tangs of the acanthrus genus, triggers, bat fish and groupers get these flukes real easy. Traditional copper meds do not fix this problem. A stronger organophosfate is needed. Fluke tabs from aquarium systems works well.