Switch too a good liquid bath soap. I never have soap scum after switching. Get a small hand squeegee and use it on the walls and glass door after bathing. After you get the tub clean, wax the fixtures with Jubilee Wax or other appliance wax. Wax the tile or surround and the soap dish or shelves and fixtures. This protects the finishes and keeps scum at bay. Re wax about every 4 months.
As far as getting scum off the stall, I use to use vinegar and baking soda mixed into a paste, let sit about ten minutes and pour hot water and scrub. It may take a few times to get it done but you will be proud of it when finished.