Blue Tang as in a hipo tang ??? Most likely Its ICh Hipo tangs and tangs in general get ich really easy. If hes eating fine best thing to do is make sure all you perameters are good and you have exelent water quality. If you have more fish make sure they aint fighting with it. Also feed nori Soaked in Garlic extract or in Selcon.
As far as treating goes Do some reading, you can treat with hipo salinity or copper, but if you do this is the display you will kill all inverts and corals, so its best to do it in another tank. But the ich will still be in the main tank unless you leave it with out fish for 3 months.
Best thing to do is like i said keep the fish happy and as long as its eating good it should be fine.