A metal polish would probably work, but I am not aware of the risks that might pose to your system if exposed to even just polish residue.
Copper "rust" is blue. If you can re-lap just enough to remove the blue stains, don't let it dry, instead use alcohol immediately after lapping to remove water from the surface. This may reduce the moisture levels on the surface, and prevent it from happening again.
Maybe, just throwing it out there.