|Uncle Jeb shows off his nictitating membranes.|
One possible diagnosis is Haw's Syndrome. In this case, the elevated lid is the only symptom present, and is often caused by an underlying parasitic or viral infection. Treatment of the underlying condition, such as with antibiotics, often takes care of the lid issue. Third eyelid protrusion can also be a symptom of conjunctivitis, in which the soft tissue of the eye becomes infected and inflamed, or of Horner's Syndrome.
The symptoms of Horner's include the elevated third eyelid (that can also be red or swollen), but also a drooping of the upper lid, constricted (narrowed) pupils, and eyes appearing "sunken." According to VCA Animal Hospitals, Horner's is a neurological problem that in cats can usually be traced to a specific incident of trauma, like a car strike or bite wound. Treating the underlying trauma can alleviate the symptoms of Horner's, which often dissipate after a few weeks even when no specific cause is found.
The important thing to remember is that an elevated and visible third eyelid usually indicates a cat is not at 100%. A visit to the vet isn't always necessary, as your cat may even just have a minor eye irritation, but if symptoms persist for more than a few days or more than one are present, a trip to the vet may be necessary. Your vet can then perform a thorough examination to rule out any serious conditions or eye diseases, and recommend a course of action if necessary.