You already have some good answers here as I’m late to the party (visiting with in-laws over the holidays), however disregard any that use vinegar or any other acid and also disregard any that suggest ammonia, unless it is a very diluted solution. Marble stains readily and the best cleaner is actually preventive maintenance. Keep it swept or dust moped regularly and wipe up any moisture immediately; wet footprints, spills of any kind. wet muddy dog tracks, etc.