M.P.
My granddaughter has eczema and once the rash heals her skin in that place is white.
Another possibility is that it's vitiglio. Here is a web site describing it. My daughter had just one white spot on her leg and this was diagnosed. I worked with a man who had a lot of white on his face and this is what it was.
http://www.medicalnewstoday.com/articles/245081.php