What disease does my fish have?

He isn't old and blind. Goldfish can live for 20 years (although most don't). If he's had it that long I would say it is from an old injury. Telescope eye goldfish can injure their eyes so I would say that's about the only thing it could be since it hasn't spread and he's had it so long. Black moors are usually fairly active so that seems sort of unusual. If you keep the water clean and he's fed properly then I don't have any other ideas. Sorry :(