The scale is soap scum, calcium and magnesium salts of the fatty acids in your soap (soap consists of sodium and potassium salts of fatty acids, which are water-soluble; calcium and magnesium, from hard water, displace the sodium and potassium, and are insoluble, so the precipitate on your brush. And in you sink...)
Before the time of syndets (shampoos and shower liquids), when people would use ordinary soap while taking a bath, baths would have a soap scum line that needed to be cleaned away every couple of weeks.
Acid will effectively dissolve soap scum. Just clean it with cleaning vinegar.