use that soft scrub stuff.

It is white and has some grainy texture, it will bleach the stuff between the stones, it shouldn’t effect the mosaic, if it is out of marble, but get a second opinion. I have had good luck with that. Is the grout still wet? its a fresh mosaic? Use a dry, pref. Microfiber cloth, wipe it off.

Is the grout still wet? You should remove excess grout before it dries hard with a damp sponge.

If not you are going to have to use a little elbow grease. You may have to use a lino knife to shave the excess off.