Marble is the most elegant material available for shower walls and is also one of the most expensive. Since it is a natural stone, it will need to be cared for in a special way so that you don't mar the beautiful surface by using the wrong soaps. Follow these steps to remove soap scum safely.
Put on your rubber gloves and wet your shower walls.
Apply an acid-free soap like MB-3 Soap Film Remover (see Resources) to your marble shower walls. Dilute the soap with 2 parts water and sponge over the surface.
Let the cleaning solution sit for about 10 minutes to allow it to break up the soap scum. You should be able to see it working.
Scrub the surface in a circular motion to remove the soap scum. After a few minutes, you should see the clean surface of the tile when you wipe away the soap.
Rinse the marble with warm water. If you still see areas with soap scum, reapply the soap in the same way.
Use a squeegee to wipe the water from the shower walls to prevent the buildup of minerals. You can polish the marble with a dry microfiber cloth (do not use window cleaner) until it has a high sheen again.