Mustard in clothing can be easily removed by running cold water over the stained area, and then using lots of dishliquid or bar soap over the stained area. The soap works to remove the proteins in the mustard and fights the discoloration. Let this sit overnight, and then launder as usual. Do not dry until you have inspected the garment in case you have to repeat this procedure.
Mustard stains that are set in, or dried in can be lightened by pouring lemon juice over the area and then letting the garment dry in the sunlight. (this only works on white clothing)
If you have rubbing alcohol handy, you can pour this over the mustard stain and then launder as usual for a great clean.
Some of us have glycerin handy in our homes, if you do, glycerin is great for getting mustard or ketchup stains out of clothing!