If you mean greyed out as in "there's no detail and it looks like a white square," then try to respring the iPad. That should fix it.
If not, go to Cydia and download the free tweak "iWipeCache." Once it's installed, start it and it'll cause a respring. Hopefully, THAT should take care of it.
Sometimes, you may have to do multiple resprings to get the white icons to go away. And, no, I really don't know the cause of them. I get them, usually after downloading something from Cydia or after theming. Again, a respring or two fixes it.
Good luck and let us know how you get on.
Marilyn