You mentioned he is one of your tortoises - are they housed separately and what substrate are you using?Attached photos of left eye and right eye
When its just one eye, it is usually trauma of some sort. Dusty dry substrate, sand, sharp ends of cypress mulch, any type of stick or debris in an outside enclosure, injury from another tortoise, etc...one of my tortoises has slight redness, swelling and moisture only in one eye. Other eye is fine. Active, eating well, no other signs or symptoms. Could it be due to physical trauma - like he bumped into something or got poked?
Thanks