Scum lines around the tub are typically due to a build up of body oils, lotions and/or chemicals from a soap product or detergent.
Bathing suits/clothing worn into the tub may carry traces of laundry detergent/fabric softener.
Shower beforehand and thoroughly rinse all clothing before entering into the tub to help prevent build up.
If you notice a scum ring forming, you should be able to remove this by wiping it off with a clean towel.
If this is proving difficult, use a soft clean cloth or sponge with mild soap and water or use 70% Isopropyl Alcohol (when the water has been drained).
Be sure to rinse well and dry with a soft clean cloth.
NOTE: Please avoid abrasive cleaners or sponges with coarse grit. These can scratch/dull the surface.