Strange marks on the back of the iPhone 5

Hi all,

recently I discovered a strange white mark on the back of my iPhone 5. I didn't drop it or anything similar. I tried to clean it but it's not a stain. It looks almost like the color would come off. Has anyone ever seen anything like this? Do you think I can get it replaced?

Here is a photo: https://www.dropbox.com/s/3vcfauq046x5ufx/photo.JPG

thank you