White patches/pale patches on the skin are pretty normal, and not usually anything you need to be really worried about. Did it just appear or have you always had it? If it just appeared, some things that might be the cause could include-
eczema (maybe even a very mild case of it)
superficial yeast infection
psoriasis
The best way to tell for sure what the cause is, would be to get to your doctor to have it checked out.