How do you remove mold from marble in a shower?

Mold will not grow on marble. Mold needs cellulose or other decaying organic matter that is not present in marble (or limestone or granite) However marble will suck in mold that is growing around it causing the marble to stain. So before you start to take the mold stain out of the stone you need to remove the mold that is causing the stain. If it is from the top or side due to build up in the grout lines you need to fully clean those out. Chlorine or bleach is the best product to kill mold, but that is harmful to marble. Try a ph neutral sanitizer. If the mold is beneath the stone in a floor and wall you have a bigger problem then stained marble. You need to remove the stone and cut out the mouldy items. At this point its probably just as cheap if not cheaper to replace the stone.

However if you want to salvage the stone you need to apply a poultice. A poultice is a mud pack of diatomaceous earth (its used in filters and can be found at any pool supply store) and water that is placed on the stone and covered with a plastic (2-3mil) sheet and tape sealed at the edges. Allow 48 hours to dry and remove. Repeat as necessary.