White stain on grout between tiles. How to clean?
To clean grout, I would start with a mixture of white vinegar and baking soda, in a 2:1 or weaker ratio.  Apply the mixture to the grout lines, let it sit, gently scrub with a soft bristle brush (old tooth brushes, for example), then rinse with clean water.  Repeat if necessary.
If that doesn't work, you then might want to try some of the more powerful commercial products.