My Green Terror has had this problem on occassion. In my case it happens when the female gets a little excited and nips at him. The lining protecting the eye has been damaged. It will heal over time. If it's too deep a bacterial infection take over in which case you can use an antibacterial medication for a couple of days. Keep them seperated until that time or she could end up losing it. Breeding can be rough especially if one isn't accepting the other.
