Use a grout cleaner that is safe for polished stone. You may also carefully scrape out the old grout with a grout scraper and reapply. Be very careful as you might slip and scratch the surface of the marble.
Michael Prachniak - Expert Tile and Stone Consultant and Writer, Owner - Tile Productions
Check out FAF''s tile section!
www.findanyfloor.com/tile/TileFloo...