I tried a magic eraser and it worked!
And here's a picture of the 18" long choose-your-size one my dad got me from Japan. I think it's pretty funny
I use Isopropyl Alcohol on a cottonball or clean cloth. Takes that marks right off. Be sure to clean out your bobbin area/casing too.
Call your machine repairman and ask him what he uses to clean marks off your machine.