A lemon slice sat on our marble bar top overnight and left a white mark. How can we remove this mark?

The acid in the lemon has etched the surface of the marble. The only option is to re-polish or hone the marble. Polish the etched area using a powder polishing compound. Work the powder into a slurry using a white pad and a slow rpm hand buffer. If the etch is light, you may be able to remove the etch by hand. In this case rub the powder into a slurry with a white pad and work in a circular pattern. Once the etch is removed apply a good quality marble paste wax to help prevent etching.