That is often referred to as "pop eye". Fish can recover from this on their own if the water conditions are stable and the fish continues to eat. It can happen if it swims too closely and rubs its eye against something sharp like the rockwork. It could be due to an alternate infection. If the fish can be transferred to a quarantine tank, it could be medicated to help it heal more quickly without risk of more damage. Matt would be the guy to ask for more information.
