Mix about 3/4 cup automatic dishwasher detergent to one gallon of water. Sponge solution on and allow to set 15 minutes, keeping surfaces wet, in order to dissolve scum. Rub with a scrub brush or white or green scrubby. Repeat applications may be necessary to removal all scum. Rinse and dry.
Common baking soda and water paste makes an effective scum remover, fights odor and mildew.
Spraying hot white vinegar on surfaces is effective on scum, sanitizes and removes odor.
For severe mineral deposits, commercial products are available to remove calcium, lime, or rust.
Whatever cleaner you decide to use, be sure to give the product or solution time to dissolve to effectively degrease shower walls and bathtubs.
Routine bathtub maintenance should minimize problems with soap scum and mineral deposits.