Could be a bacterial infection, popeye from fighting or possibley netting damage. It can also occure due to high DOC
's in the water. As long as the eye is not severely bulging it should be easily rid of with a few large water changes over the next week and fortifying the foods with some Zoe or Selcon daily.
If it does not clear up on it's own after attempting to remedy it naturally, you may need to quarantine and treat with an anti-biotic. I would urge you to try the first option. Often capturing and moving the fish can cause further damage, possibley causing the loss of the eye.
Have you noticed any aggression between the angel and the foxface?