White stain on grout between tiles. How to clean?

The white substance is minerals that were deposited when water evaporated. This can easily be removed in several ways. The fastest and easiest is to purchase a de-scaler (Lime-a-way is one brand name). Simply scrub it on with a brush and watch the scale fizz away!

If you'd rather use a less harsh product any citric acid-type liquid will work (lemon juice in a concentrate). Or white vinegar, but it will take much longer.

Which ever product you use wipe down the cleaned area with a wet sponge or rag to remove any residual descaling/citric acid which could weaken the grout joints.

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.