Bar Keeper's Friend is the best way to clean soap scum. However, you can make things easier on yourself by preventing soap scum in the first place. Get a squeegee and after EVERY shower, squeegee all the excess water off. Then spray doors with a soap scum preventer. You can also switch to a non-creamy body wash instead of bar soap which has a lot of binders that lead to soap scum in the first place.

Barkeepers Friend will damage your marble...vinegar works on soap scum...but ruins marble....actually I think you are down to good old elbow grease and a scotch scrubbie. Alcohol won't hurt the marble....and may help with the soap scum.

What is a soap scum preventer? Brand?

The Barkeepers Friend is only for the glass shower doors as I thought from the OP that the soap scum was on the doors. As for the soap scum preventer, you can use Tilex Daily Shower Cleaner and spray every day after you squeegee.