I had the same thing happen to a Nasso that I have a while back, I think it was from scraping on the rock, most likely bullying from my Powder Blue. The Nasso had several of these marks, I eventually transferred him to my sump, the marks disappeared within a few days and he was given a new home. Do you have any other fish that may be bullying him?
One of a few marks that showed up on my Nasso (just behind the eye)
HTH