That fish is a yellow tang. It usually costs anywhere from $20-$40.
It should be fed a variety of foods, mainly algae and vegetable based. I would recommend picking up some freeze-dried seaweed/nori, and then 3 or 4 different frozen foods such as mysis shrimp, prawn, clam, kelp, krill, etc. The best way to go is to get a few "formula" foods, which combine a bunch of ingriedients to make one mix food. Ocean Nutrition makes formula foods such as Formula One, Two, Prime Reef, etc. These are best because they give the fish a very wide variety.
More live rock is also certainly recommended because yellow tangs are a grazing fish. They pick at the algae on the rocks and use the live rock for protection. Furthermore, live rock helps to maintain a more stable and healthy enviroment for your animals because it acts as a natural filter.
I would also highly recommend that you slow down on the fish additions. You stated in another thread that the tank was just set-up last week and you already have four fish in the tank already. You are soon headed for disaster if you do not stop adding fish. Following the rule of adding one fish every 3 to 4 weeks from here on out would benefit your tank greatly and monitor your water levels very closely because you may see a spike in your ammonia/nitrite/nitrates very soon and this could be very bad for your whole tank.
Good luck, and ask any and all questions you may have!