The "third lid" is usually a sign of a health problem. Are his/her eyes droopy, excreting discharge? This can be a sign of an allergy, an upper respiratory infection, dehydration, etc. I would get kitty to the vets as soon as possible, and hopefully if it was caught soon enough, some medication, either ointment for the eye or oral antibiotics, can be given and treat it completely.
Bottom line, it's time for a trip to the vet. Good luck and I hope kitty feels better soon!
Bottom line, it's time for a trip to the vet. Good luck and I hope kitty feels better soon!