This is a strange one for me, as before the framework laptop I had my 7+ years MacBook Pro (2015) and never had anything similar.
Somehow, the bottom cover started to “fade” in the area where I keep my hand.
Did anyone have similar issues with the cover/trackpad?
Other than that, I love this laptop 
So what laptop is that ?
Not a 13"
EDIT: update
OK so that’s a 16" Maybe edit the title
Done that for you 
We crafted the Framework Laptop 16 chassis from high performance materials like thixomolded magnesium alloy and machined aluminum.
As the chassis is metal I can only think you have salt from sweaty hands that acts as a corrosive. You ‘should’ be able to clean it; I can’t imagine it’s dirt or dye.
See this topic maybe too
Hi,
Thanks for updating the title.
Unfortunately, I don’t think that’s the same as is quite smooth and I couldn’t clean it.
Also, that’s the only area, so is not where I keep my hand while typing.
The only other laptop I can compare with is my old MacBook, and I didn’t have anything similar on that one.
I believe the bottom cover on the 16" is painted magnesium, so it sounds like the paint may be wearing off in that spot.
That makes sense then.
A bit strange, as that’s the only spot and I haven’t used the laptop too much.
Almost all the newer MacBooks are a machined basically straight aluminum; no painting, coatings, etc. The primary function was the conduction of heat to keep the noise from fans to a minimum. Other benefits are:
That spot in your picture does look like paint worn away, or maybe it was anodized and reacted with something (skin oils, etc.) starting on the edge. 
