Summarize this article:

The third eyelid is white or light pink. If it is protruding, it will come out and at least partially cover the cornea (clear covering of the eye). As long as the third eyelid is covering less than 50% of the cornea, your cat will still be able to see fairly well.  Haw syndrome is a neurologic condition in which both third eyelids protrude.  Horner’s syndrome, another neurologic condition, can also cause third eyelid protrusion.  The third eyelid has its own tear gland. Instead of seeing the third eyelid itself, you may see its tear gland protruding. This condition is called cherry eye, which is rare in cats. The gland will look like a pink, oval mass. A protruding third eyelid is not always abnormal. For example, it will protrude when your cat is sleeping deeply and retract back when she wakes up. It will also come out if she gets into a cat fight—a small eye muscle will pull the eye back into the socket, allowing space for the third eyelid to come out. If the third eyelid protrudes when eye protection is not needed, then the protrusion is likely abnormal. Causes of abnormal protrusion include:  Significant weight loss or dehydration, causing the eye to sink back into the eye socket Third eyelid tumor or inflammation A mass within the eye, pushing the third eyelid forward Neurologic problem (Haw or Horner’s syndrome) affecting nerve control of third eyelid If your cat’s third eyelids are protruding, her eyes may become red from irritation. For example, if the third eyelid’s tear gland is protruding, it may become red from airborne dust. Dust could also cause the third eyelid itself to become irritated and red. Third eyelid protrusion in cats can have many causes. Your vet will need to perform a detailed eye examination and perform some other diagnostic tests to determine the protrusion’s cause. During the eye exam, your vet will examine the protruded third eyelid, measure tear production (Schirmer tear test), test the pupils’ response to light (pupillary light reflex), and use a fluorescent green stain to look for injuries to the cornea.  If your vet suspects a neurological cause, they may want to perform other diagnostic testing, including a neurologic exam and skull x-rays.  Over-the-counter (OTC) eye treatments for people don’t work well in cats. Do not treat your cat’s eyes with OTC treatments before taking her to your vet. Doing so could make it more difficult for your vet to diagnose the protrusion. If your cat suffered an eye injury, take her to your vet immediately for treatment.

Summary:
Look for the third eyelid. Remember when you see the eyelid protrusion. Check your cat’s eyes for redness. Take your cat to your vet.