Having been in the fire and water restoration business for over 30 years I have a few tricks up my sleeve!

Remember, like kind removes like kind.
ie - oil base stains are removed with petroleum based products. etc.

Mustard is one of the hardest stains to be removed, but can be done...if you haven't already set the stain using the wrong product or putting garment in the dryer. Don't use detergents.

When you get a mustard stain on whatever.... the first thing you should do is gently remove as much of the excess as possible. Don't rub. You will open the fibers and probably never remove it completely. You can then rinse from behind stain with cold water, only cold. If you will then open the exact same container it came from and just try to skim the oily/watery substance out, put it on the affected area for a few minutes and then rinse with cold water you should be able to remove it. Or at least most of it. If you need to, repeat process until it's completely out. You must use the same container the mustard came from.

This process will need to be repeated sometimes for complete removal, depending on a few factors.
1) How long has the stain been there?
2) Has it dried?
3) How much mustard?

Tip

Always use cold water when removing the fresh stain and let chemical sit a while to loosen stain. The chemical sitting to loosen stain is called , "dwell time".

Until my next tip...Happy mustard removing and happy Steeming...for this...cold Steeming!