The way I found that works best for me to get dried mustard stains out of clothing is to use some white vinegar, water and dish soap to remove the dried mustard stains from clothes.
Take a cup or bowl and fill it with half cup of water and a half cup of vinegar and then a few drops of dish soap and mix the solution together.
Then take a sponge or washcloth and soak it in the solution and then dab the mustard stain with the solution and then allow it to dry.
You can also put a sponge soaked with the solution under the clothes with the stain which will help it soak through and pull the stain out of the clothes.
Then dab a little more of the solution onto the mustard stains and then gently scrub it out and you may have to use a toothbrush to help scrub the stain out.
Then place the clothes into the washing machine and add a cup of vinegar and baking soda and let it sit for an hour in the washing machine and then start the washing machine to let the mustard stain wash out.
If that doesn't work then you may need to use some Resolve Carpet Cleaner or Resolve Stain Remover for clothes.
But the vinegar, dish soap and water usually works for me to remove mustard and other stains that have dried out.