The cause can be related to tramua-usually happens in one eye. I also believe bacterial (poor water quality esc..)
The treatment, would be to maintain good water quality and supplement their diet with vitamins just to help boost the immune system.
If you keep up on water quality (and give them a good diet) it will heal by itself.
this is one of the times the fish with an aliment should stay in your tank as you don't want to injure the eye anymore then it is.