Unless the eye was damaged somehow, it's most likely a type of bacterial infection. It can be treated with anti-bacterial medications, so set up a hospital tank asap and keep a close eye on the rest of the fish.
These types of infections infrequently come with livestock. Most often the result of is from poor water quality, so make sure you're filtation and maintenance routine is keeping the tank pristine. The water quality of a heavily stocked and decorated mbuna can decline rapidly.