One eye closed can be a sign of a lot of things. Take it as an early warning of something wrong systemically, not just the eye itself. Direct injury, dehydration, vitamin deficiency, stress, infection. Check for any other signs that he's not acting normally. You can loosen any sticky material on the lid and ease the eye by flushing the lid with sterile saline (over the counter contact lens solution). Don't put anything in the eye itself until you know exactly what's wrong (actual injury or infection). But, use this to check over your husbandry...temps, humidity, vitamin and mineral supplements, gutloading, correct lighting, etc.