[Kevomac]

Don't forget pallette tang, as another common name. My regal blue tang (as I like to call him) has visible white "teeth". His partner's "teeth" turned pink before he died. White seems normal to me.

As for the fish in question, looks like stress. Could be being harrassed, could be water params, could be because Hepatus (sp?) tangs are shy and stress out easily. Small ones seem to be more prone to stress, as mentioned, but these fish do tend to freak out easily and can die from it. Hopefully yours will be okay, though. You might try increasing his feeding (I find sushi nori is good for them) and you might also try isolating him. Remember that tangs eat primarily greenery which does not have as much nutrional value as meaty food, thus they have to eat a lot more to get the nutrition they need.

HTH!