yellow bellied turtle, eyes have turned white

by michelle
(england)

i have 2 yellow bellied turtles which i have had for a few months now. After cleaning them out and feeding them some live shrimp i have noticed one of them has white bottom eyelids which keep closing over the whole of both his eyes,this seems to annoy him as he trys to rub his eyes.when his eyes are open they are clear and look normal.have never seen this before and don't know why it has happened, hope you can help.