I would have to agree with the idea of mechanical injury to the eye. I do not know if the tank mates are aggressive. If not I would observe only. Make sure he feeds well.
I would prepare for treatment of secondary infections with antibiotics as a second resort only , and if, in a hospital tank.
It sounds silly, but I have a nightlight on for my fish (Kessil), as I lost one fish..dead in the mornings, no reason others than possibly nighttime trauma.