OP:
You have a condition known as "StainGate".
If you've never heard of it, you had better "read up" on it.
This can't be "fixed", except by the following.
Only solutions are:
1. replacement of the display (this MIGHT be covered by Apple's 4-year "StainGate replacement program", you haven't told us how old the MB is)
or...
2. removal of the entire sprayed-on anti-glare coating (will look better afterwards).