I believe that it's a genetic defect from birth. Some fish are born with one eye as well. I was at a lfs and had this nice Midas someone donated and it had one eye. Like no socket nothing it was pretty cool lol he was pushing 7-8 inches.
as stated, it's a birth defect. He's SUPER thick already... like 4.5" tl and already a full inch thick. I'm concerned he may deform more as he grows, but for now he's fine and definitely my favorite fish