My walker had this happen a few weeks ago in both eyes. I kept wiping it out but it kept coming back. After a few days he started acting very lethargic and I could tell he was very sick. Wouldn't even get out of his dog house when I went to feed him. Had to hold the bowl in front of his face for him to eat.
I honestly thought he was about to die. I started injecting him with a cc of penicillin for 7 days. After the 5th day he was back up and acting normal. I gave him a small dose of b12 too. I went the cheap route though and it worked. Just didn't have the money for a vet bill.
Maybe he just has a cold or whatever. But if its just that one eye it could be something different and may cause more problems later on. Taking him to the vet would be your best bet