Was the area dry and clear of mold when it was installed?

Keep it clean and dry, so mold doesn't grow. Consult the manufacturer for recommended cleaning products and use that exclusively. Use a squeegee or a towel on the tiles and shower doors after showering to remove excess water. Air circulation will keep things from staying damp so open a window or turn on the exhaust fan, and leave the shower doors and the bathroom door open so it can completely dry out after each use.

Here is simple but effective method! Mix together 1 qt. of warm water and 3 tbsp. of baking soda in a container. Baking soda is a powerful natural cleaning agent that can remove mold without damaging marble in the shower.

Squeegee immediately, place a fan a few feet away from shower and let blow directly into the shower for several hours after use. I keep a fan blowing all night after night showers. Be sure and keep bathroom door open to allow all moisture to escape from the room.

Our marble showers are about 12 years or older and so far no trace of mold.