I'm no expert, but one of my LFS
guys told me once that fish commonly get abrasions on their eyes during capture and transport because of no eye lid. And these abrasions can get a bacterial infection. Did you say if you've acquired him recently? And if its only 1 eye, that's what i would guess it is. I think there are antibacterials you can buy for the tank. Again, this is just my estimation of what it might be.