I guess I did forget all that, didn't I?
I am feeding him Ocean Nutrition Prime Reef Flakes, which is the same thing the pet store was feeding him when we bought him over 3 years ago. I haven't changed it at all.
His only fishy companion is Black Fiffy, the 3-stripe damsel, with whom he has lived since we brought him home. There are a couple of hermits and also a horseshoe crab. All these have been in there, nothing has been changed in the last 3 months since I have noticed this issue.
He has lived in a 10 gallon tank. I will have to re-run the parameters, I need to find my log anyway, I haven't checked in the last 3 days.
That is the biggest mystery - I could see issues if I had added anything or changed anything, but nothing has changed in months. I did at one time add another fish, but the damsel killed it and that was over a year ago, way before this issue started. I am just wondering if it isn't old age? The LFS I bought him from said that he was in their display tank for a long time, and they broke it down to re-do and add more different fish - we bought him at a real deal, $10, and have had him ever since.