Assuming it is a cement based grout, try some plain vinegar...it will disolve the calcium. Limeaway may do it. This shouldn't hurt a plastic base, but check the manufacturer's cautions or warnings to see if a mild acid is a problem. If it is a sanded grout, be careful about rubbing...the sand you wash out will scratch the plastic. Maybe take a paper towel, saturate it with the liquid and let it sit overnight covered with some plastic so it doesn't evaporate. I do that if I didn't clean my mixing bucket really well. I'll have a slurry of sand and cement particles by morning.