My customer has a new marble floor that has a white powder on it. It cleans up with water and soap, but keeps coming back. Is there anything I can use to clean it off permanently?

This a common condition, especially if the floor is new. When the marble is installed, it is set in a mortar base that is wet; as the moisture leaves the mortar, it carries with it dissolved salts which are deposited on the surface of the stone. This condition, called efflorescence, is what you are seeing as a white powder. Unfortunately, every time you clean the floor with water you are only dissolving more salt in the stone, and when the water dries, the salts return. To correct this problem, do not mop the floor with water. Either vacuum the powder or buff it off with a dry pad. The salts will eventually work themselves completely out of the stone. If you must wet-mop the floor, use a stone soap with cold water and wring the mop out tight, making sure to avoid puddling on the floor.