Looks like a bruise to me. If he's a super active swimmer its likely he could have brushed against the rock or something or bonked into the glass.
Bit of info and a pic here:
http://www.wetwebmedia.com/nasoldis.htm just a short way down the page there is a photo and a post called "Black Mark on Naso Tang?" with a photo.
Needs to be fed more, try some meaty foods to get some protein in him.