Any chance of getting pic with white light? One trick might be to turn off your blue tank lights and have somebody hold a flashlight on the fish while you take the pic.
Even with that, I may not be able to tell you what that is. Have you seen any negative interactions between the foxface and another fish, say the yellow tang? Those spots could be from two scalpel hits from a tang that have gotten infected.
Do the spots look eroded down into the skin or are they superficial at the skin surface?
My first thought was a Tang too, recently added a large Scopas and a Hippo. They all seem to be getting along fine though. I'll try to get better pictures under better light, that Fiji has always been skittish though, every time I pull out the camera he hides.
Oddly enough the spots seemed to have disappeared as fast as they appeared. I can still see them if I look close, but they are 95% gone. Fish seems fine.