Eye problems are some-what common in chameleons. Sometimes if it's just one eye it could be a foreign object in they eye. Sometimes a good misting can help get it out but other times a small catheter or syringe will help flush it out. Don't do this on your own! You should have a list of vets that treat reptiles in your area on hand at all times right
If everything else looks good, eating, behavior, color etc. that would be my guess. However if left unattended a foreign matter in the eye could be fatal. malnutrition can be another cause but I am guessing you have been providing all it needs, UVB, heat, misting, good food. Remember they do need a basking area from 90-105 with ambient temps at about 80 and in the 70's at night. Without the proper heat they can develope respiratory and eye infections. I see alot of people forgetting that veileds need a basking spot and just keep an ambient temp. I am sure you know all this but I just thought I would go over it. I wish you the best of luck and hope your little guy/girl gets well soon.