My PA mono of over 20+ years old also has a small patch of white that couldn’t be wiped off and I only see it when putting it at certain angle under the light. I got it new from boutique so don’t know how or why it’s there. The only thing I could think of that it could be caused by moulds(?) as it‘s hardly been used and in the dust bag forgotten somewhere in the closet for many years. This also happened to my Bucket bag even worse than this and the lining was peeling off and sticky too.
It looks like the canvas is wearing off. Personally if that wasn't disclosed I would return it. Hopefully you didn't pay too much for it. It looks like damage caused by improper use. I have a DE speedy from 2010 and the canvas is perfect.