I planted cucumber plants a week ago and now the leaves are turning white. Is this normal?

It could be what they call "chilling injury".

"temperatures in the 40s and low 50s can cause chilling injury."

Does that article help?

I also found that there are diseases that result in the leaves having white "stuff" on them, but it sounds like you're saying the leaves themselves are white - so probably not disease from what I could find.