Help figuring out what's wrong with my koi

One of my koi has had a bump over his eye for about 2 years; first noticed opening the pond after the winter. It seemed to be a lesion, open but not bleeding. We thought it may have bumped into the heater or an animal had scratched it. It looked recently as if it was getting better - smaller and closed over. This morning, that's how it looked. Later this morning, it seemed to have "ruptured" returning to how it used to look. Here are pictures of what it looks like currently (note: not the most pleasant thing to look at): http://imgur.com/a/7QDLe

Any ideas anyone has of what this might be would be helpful. We'd like to do something to help it if possible, as well as to help the other fish if this could impact them at some point.