Is he blinking excessively? Rubbing at it?
Has he been sneezing lately?
Sitting near a dusty heater? Or been outside recently?
Definitely not the third eyelid as that's actually pink/whiteish & not transparent.
Looks like it could be a bit of excess mucus in the eye but it's always a good idea to have any concern relating to the eyes checked out.
Avoid biting when a simple growl will do
The Spirit Lives As Long As Someone Who Lives Remembers You - Navaho Saying
Vindication ~ For all those pets who became sick or lost their lives from tainted pet food