White vinegar will get it off, but don't use it very often, because vinegar also dissolves calcium, which is a component of the grouting. Once or twice a year won't hurt, but every week, no.. unless you just love to regrout. You can also unscrew your showerhead and soak it in a solution of 50% vinegar and 50% water, and then use an old toothbrush to clean all the white (calcium) deposits off.