Soak the stained area with a heavy duty liquid clothing detergent. Let the detergent soak into the stain for five minutes, but don't scrub or attempt to work the detergent into the mustard stain.
Combine 2 tbsp. oxygenated bleach powder with 2 cups of water. Soak the stained area of the fabric in the solution for 15 minutes.
Combine 1 tbsp. chlorine bleach with 2 cups water if the stain persists.
Soak the fabric in the bleach water for up to 15 minutes. Do not exceed 15 minutes of soaking, as the bleach will weaken the fabric fibers. Mustard that fails to fade within 15 minutes cannot be removed.
Wash the fabric immediately after bleaching. Follow the washing instructions as detailed on the fabric's care label.