Help!! What is happening to my Naso Tang?

[newreefer_59]

Purchased this 2 1/2" Naso Tang 3 weeks ago. First week, he had small white specks (perhaps ick or what-not) but all alse seemed fine. Last few days, however, he has developed the darkish scuff mark on the underside below mouth and gills and is even a little larger today!? He is eating very well and not at all behaving like he is ill, but wondering what the heck this is and is it something I can cure.

Any advice is appreciated.