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It sounds like you are seeing the third eyelid. The third eyelid will present iself when there has been a trauma to the eye or the dog has conjunctivis. You will also see it if the dog is suffering from allergies. Parasites can even cause it.
You can flush the eye with warm saline to remove any debris that might be the cause. If this does not help, you will need to see the vet for an exam and possible antibiotics.
Let me know if you still have questions.
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.