Could be a little scratch from another cat or a bush and will hopefully clear up on its own.
Just one thing I'd like to mention, in the spring Minx had red looking eyes; the vet diagnosed conjuctivitis and gave me drops to treat it (fun, fun, fun

). However it was so difficult getting the drops into her eyes and we were both stressing out. After a bit of research I found her symptoms fitted the feline herpes type 1 virus, it's basically a cat version of the human cold sore virus. The symptoms were one of her eyes looked a bit red, she would wink a bit and the discharge looked dark brown, then as one eye seemed to get better the other would flare up. Otherwise she was fine, eating, playing etc.
I'd seen a food supplement called L-lysine recommended for treatment of the herpes virus, but kept dithering until I found out lysine is also present in tuna and I noticed that her eyes seemed to improve when she'd had tuna.
I ended up buying a food supplement for cats called Enisyl-F - must admit I was nervous about using it as I hadn't consulted the vet first, but after another few days of her eyes improving/relapsing I started adding it to her wet food and within a matter of days the condition had cleared up. Obviously this isn't the answer for all eye conditions, but it made life so much easier than treating her with the eye drops that only seemed to provide temporary relief anyway. If it's necessary to take your cat to the vet for his eye it might be worth bearing in mind to mention.