Im gonna guess he ran into something. Seen it before. The eye will totally cloud over, you won't be able to see the pupal, then it will either heal up, if the damage wasn't big, or when it clears the eye will be gone.
I don't want to freak you out anymore, but I had it happen to one of my fish, and when it was done healing, you could see into his head. He was just fine, just missing an eye.