About 15 years ago, a customer asked me to take a look at one of his goldfish and tell him what I thought. It looked identical to your fish, but was a red-orange colour. The fish had been like that for 3 or 4 years, and was eating well and acting as if nothing was wrong. He (the customer) came back the following year to tell me the fish had died, but was still eating the evening before. My thought is, it's probably a tumour of some sort, but the fish would have to be sacrificed to find out, so enjoy it while it lives.
John