Been there...
Do you have any LPSs like a torch or frogspawn? Most likely it is getting stung by it. Watch him/her for the next week or so. You will see some spots fade and others appear.
-Karen
May sound wierd, but he's always glued to that platygyra in the pic. I wonder if it stung him at night? I'm gonna keep an eye on him. I'm just hoping its not some disease.