I just ground a granite floor and the grout is getting a white powdery residue on the surface of the grout.

This is a condition known as efflorescence. There are soluble salts in the grout coming to the surface. The large amount of water you used dissolved the salts and they are depositing on the surface. Efflorescence will eventually disappear when the floor and the grout is completely dry. Wait several days and mop the grout joints with stone soap and very little water. Avoid wetting the floor too much, since that will only dissolve more salts causing further efflorescence.