Is my yellow tang eating?

I just got my yellow tang. So far he is swimming around the tank very nicely and has been it looks like nibbling on the live rocks. I just started my aquarium about 2 weeks ago so I o not think that their is a lot of algae yet. I just want to make sure that he is actually eating. How do I check this, and what other foods should I feed him? I just put some brine shrimp in there but all he did was chase it and then stop. Is he eating okay?