Do you mean yellow tang? if so then the spots might be ich. Tangs in general gets ich easily in the first few weeks of acclimating to your tank. To keep it healthy feed it some garlic enriched foods and make sure water is super clean. It's possible for the spots to spread to other fishes so if you have a q-tank it might be a good idea to keep the fish in there until ich disappears. Also some people have reported good results with kick ich(medication sold in some lfs and online stores). Good luck.