Palm Rest Discoloration

Hello,

I am using a 13-inch MacBook Pro Retina (late '13). About three years ago, I started noticing small black dots on my MacBook's palm rest. The dots were not immediately conspicuous due to their size, but noticeable enough that I decided to use a palm rest protector. Today, I pealed off the palm rest on my MackBook, only to be greeted by the discolorations again.

I am curious to know what could have triggered the discolorations, what solutions there might be to prevent their growth, and what ways there might be to possibly remove them (although I doubt this would be possible with the skills of a layman). Thanks.