USS Eagles - Nictitating membrane

Definition: a whitish or translucent membrane that forms an inner eyelid in birds, reptiles, and some mammals. It can be drawn across the eye to protect it from dust and keep it moist.Also called third eyelid.

You can see a bug flying around Dad's eye, and then the membrane doing it's job of cleaning off the outer eye. A nice close-up view of it in action.