My Chai has the same problem. The vet has been treating it with prescription eyedrops and baytril for some sort of upper respiratory infection that tends to manifest in and around the eyes. It could also be that he scratched himself... but I suppose that would be more obvious.
I took in a rat who had an ear infection, and she was producing porphyrin from one eye (the side with the infected ear). I recommend sniffing the ears to make sure they don't smell foul. If you smell *anything*, see a vet asap. Also look to see if you notice the rat digging in that ear, that can indicate discomfort.
It could also be a corneal abrasion, which is not a big deal in most cases. But it's not always obvious.
One of my boys has had this the whole time I've had him. He's been to the vet for it more than once and we were never able to find a real diagnosis or solution. He's been on eyedrops and he's been on Baytril. Nothing has worked. He was also checked for a corneal ulcer, and it was all clear. My vet eventually got stumped. She thought maybe it was just chronic leakage from stress or something.
Try steeping a green tea bag until it's warm and drain the packet so it's just damp and lightly clean the porphorin eye with it twice a day for a few days and see if that helps it. A girl on YouTube posted a video about it and I found it rather interesting. I believe her name is RatGirl44?