My mom does a lot of work with animals and I showed her that picture and she says it's an ulceration and I'll let her type out some stuff she says can help since I don't know much about this kind of thing.
Hiya! Your all purpose rentamom here. I work in animal rescue and have been treating non-emergency ailments and injuries in animals for several decades. Firstly, thank you so much for helping this cat. so many strays don't have anyone to care for them, so you reaching out to her is so admirable. From the pic the eye appears to have become ulcerated. This can occur for a number of reasons, but virus or injury are your best bet. Since she's timid around the bush, injury may well be the cause. For treatment, you're going to need to make sure you wear gloves and a thick shirt since she might decide to perforate you. Any scratches you hopefully wont receive need to be cleaned really well, as ferals can sport some nasty cooties. If you can get a h'old of her, the eye needs to be well rinsed. You can use one part water to one part peroxide for this. If you can order from Amazon US, I suggest a product called Veterimicin Antimicrobial Pink Eye Spray. This way you can just sneak up like the ninja I know you to be and squirt it right in her eye if you can't handle her. It's incredibly effective with this type of injury. And if you can actually hold her, there is an antibiotic slave (also predictably available on Amazon) called Terramycin. It's a very effective opthalmic ointment that you apply directly to the eye itself with your finger. If you can get laxtex gloves to use, that's ideal. I admit fully to having caught unspeakable eye infections when working with animals. Wash your hands well afterwards. You need to make sure to watch the eye carefully. With some corneal wounds, swelling can occur rapidly, often causing the animal to lose the eye. And I do warn you that while you may be able to save the cat's eye from further damage, it might sadly suffer sight loss. Still, better a tad blind than dead. I hope this doesn't ramble too horribly. If you need to ask anything else, please don't hesitate. Good luck and thank you again for caring about this cat. And now, back to my sprog.
Dang that's a lot more writing that I was expecting her to do but I guess more info is better than less, hopefully it helps.