In some purebreds and domestic cats with long angular faces (like Oriental Shorthairs for example) the third eyelid tends to be slightly visible in the corner of the eye for most of the time. My Sonic also has a slightly 'lazy' 3rd eyelid in his right eye due to a bout of Horner's syndrome (inflammation of the nerve that controls it) some months back now - vet says all is well now but it is more visible than the left eye when he is tired.
In general though, if it is partially across the eye it is a sign of ill-health - eye irritation, dehydration, fever, or serious worm infestation are some of the more common causes.
How does your cat seem otherwise? Any lethargy or loss of appetite? Is this something new or has it always been a bit visible?