I meant cat. Sorry. I have answered so many question tonight that I lost track.
My post was for a cat. It does sound like you are seeing the third eyelid. In feral cats, the first thing that we treat is a parasite problem. This is step one. Since you can't catch her, you need to start with the easiest thing to treat. I suggest that you worm her with a medication like drontal or droncit that you can get from your vet. Hide the pill in food and be sure that SHE eats it. Then re-evaluate it in a couple of days. If this fails to work, you will need to catch the cat in a trap. I can tell you more about that if you get to that point.
I manage several colonies so I know what you are going through.
And sorry about my mix up with the dog/cat thing. I hope you understand.