The third eye-lids are a pair of nictitating membrane that hiding in the eye
corners of a healthy cat. Once they show up, in general, it could mean
something is not very right, even round worm can cause it. There are several
points:
1. it is not necessarily a problem with the eyes, so unless confirmed by
your vet, don't treat with any eye-meds. Check if there are excessive
secretion from the eyes, which is the typical sign of eye problem.
2. ask your vet if there are noticeable sign of p