Had the exact same thing happen w/ this exact fish. Unfortunately I was dumb enough to keep this fish in a 5 gal nano tank. The white marks developed after about 1 month of being in the tank. When I moved him to 46gal tank, these white marks seemed to vanish. My original thoughts were that he may have been stung by my frogspawn and/or hammercoral, but looking back on it, I think that stress may have been at play (due to the severely undersized tank he was in).