My turtle has a white shell how do you treat it is it a problem?

This is something that needs pictures and more information. Do you have a basking spot for your turtle? Why species of turtle is it? How much water does it live in? Does your water have a lot chlorine? How old is it? Is it an albino turtle? Is your turtle wheezing or have mucus coming out of its eyes or nose?

There is so much information needed before anyone can even begin to answer that question for you.