Blot the fresh mustard stain with paper towels. If the mustard has dried, scrape it off with a butter knife.
Apply glycerin to the stain with your fingers. Rub it over the entire mustard stain and allow it to sit on the stain for up to one hour. Blot the stain with paper towels to remove the glycerin and to lift the stain.
Rinse the mustard stain under cool water to remove the mustard from the surface.
Launder the denim in the washing machine with hot water and follow the instructions according to the care label. Apply color-safe bleach to the washing machine.
Hang the denim to dry; the heat from your dryer can make the stain permanent. If you still notice the mustard stain, apply a commercial stain pretreatment that is color-safe according to the manufacturer's instructions.