Well thats definitely different. Looks otherwise healthy though as a fish.I havent had exactly that but I have had fish get injured and the eye turn whitish and lose the ability to see. But wasnt quite the same and it happened over days to weeks. Not months like yours. . The fish in question eventually got pop-eye in that eye and died. Im pretty sure it was all bacterial related in my fishes case from the injury.
How old is the fish?
Al