The pupil is just a hole in the iris, so if it is actually the pupil that looks green, then you are seeing something that's going on in the back of the eye. Cataracts usually look white or milky through the pupil, not green. The back of a cat's eye is very bright, so it is normal for their eyes to glow bright yellow-green when a light reflects off the back of the eye, especially in dim light when the pupils are dilated (think the bright eyes of a cat (or other animal) on the side of the road at night). If you're seeing bright green through the pupil that is definitely more so compared to the other eye, and in normal lighting, you might be seeing hyperreflectivity of the fundus (the back of the eye on which the retina sits), which may indicate inflammation in the back of the eye. Are both pupils equal in size to each other? And when you shine a light on the "weird" pupil (ie with a small flashlight or penlight), does it shrink down normally? Is the "white" of that eye more red than the other normal-looking eye? Does it look/feel swollen or is it painful when you touch the outside of the eyelid? In any case, I would still take your cat to his veterinarian, so that the back of his eye can be looked at and possibly other tests done.
Good luck and keep us posted!
