Ok.
Do you have an issolation tank, and antibiotics from the doctor.
So, this thread is almost exactly 1 year old. Was there ever any determination on what the problem was? Did the medication work?
I have exactly this same thing on one of my GBRs. I've never seen anything like it. All the other fish in the tank are 100% spot free; but, this one gal has a spot on his tail fin and one on her dorsal fin (instead of pectoral fin). Trying to identify it is driving me right round the bend.
((hoping the original poster will read this))