Welcome. It is normal and usually goes away once you turn the page. You don't have to worry that it is burning on your screen like a computer. The first time I noticed it I was worried as well. If after you refresh your screen it is still there then I would call CS.

This bugged the daylights out of me when I got my K2. I remember the old monitors and terminals from 25-30 years ago. They burned in very badly, and it was permanent. After a day or two, I realized that the Kindle "burn in" was only transient.