Purple Tang whitish area

[mwmp5]

I noticed an area sort of whitish on my 4 inch Purple Tang. It's on both sides of the body in front of the tail fin. Looks like a smudge teh size of a small finger print. It is not white spots (doesn't look like ich). Fish seems healthy, swimming well, and eating. Anyone know what it is? I hate to stress him out by trying to capture hime for a fresh water dip if I don't need to.

 

[RBU1]

need a picture.

 

[mwmp5]

sorry about that, hoping the description might match up to something. Will try to get one of him.