This happened to my grandpa at the airport before flying due to his high blood pressure. It just happened near the bottom of his eye, not covering the cornea like with yor rat. In my grandpa's case, his blood pressure rose from it's already high level to an even higher point due to the stress of flying, causing a blood vessel in his eye to rupture. Once his pressure normalised, the vessel healed and his eye gradually cleared up by itself, iirc. He actually didn't feel it at all, we had to point it out to him, so your rat may not be in any pain, but only "seeing red", or rather a darker shade of black and white. From what you describe, it's not really bothering him and there are no other symptoms, which is consitent with the high blood pressure option. I don't know how to treat or even check for high blood pressure in rats, and you should visit your vet to have this looked into further, but my best guess is that this is the cause of the bleeding. So if you treated the blood pressure (if that's what's wrong), you'd solve the eye problem. Maybe you could help it by setting it apart from the other rats for a while (if there is a competitive or stressful dynamic) or cuddling it quietly by itself more often, and also researching diet changes that would lower its blood pressure.
Of course, as others have mentioned, watch for any other signs that could signal something else that's wrong! Good luck!