This could be several things. The easiest would be mucus from having dry eye(keratoconjuntivitis sicca). However if this material appears to be on the surface of the eye and fixed it is most likely something else.
There is a third eyelid called the nictatans that may be protruding. This can be mainly white. It will protrude for many reasons. When it does it is called Haws
A healing ulcer on the eye often produces scar tissue, which can be dense and white sometimes with little blood vessels feeding into it. This can sometime look like a small island on the surface of the corneas.
There is also a condition called eosinophilic conjunctivitis that can cause a vegetative growth on the surface of the eye.
I can not see what is going on so I can only guess and give you ideas of what it could be.
You really should have your kitties examined by your veterinarian to make a true diagnosis of this. Some of these conditions can lead to blindness if left untreated.