Wow! Definitely got something going on there. Any chance you can trap him and get him to be seen by your Vet, or better yet an animal opthomologist? If not, can you have your Vet take a look at this picture to give you their thoughts? Is he acting strange in any way? Obviously this eye is also exuding some gunk, but I don't think that's the least of his worries, frankly.
I don't even think it's a matter of the one pupil being larger than the other (which can be the result of trauma) since it's so off center. And it doesn't look like any of the Iris Melanoma's I've seen, so I'm truly stumped.
(edited to correct oncologist to opthomologist above!)
It might be the result of an injury. it doesn’t look inflamed at this point, so whatever it is is probably well-healed and the cat may have good vision in that eye. My Armani’s eye looks worse and his vision (checked by ophthalmology) is excellent. Do show the photo to a vet, they have the greatest experience.