Mustard makes a wetter stain than you might think, so it's important to blot it up quickly to keep it from spreading. Once you've blotted up most of the liquid, scrape up any lingering solid bits. Mustard acts as a dye on fabric, so it's particularly tough to get out. Your best bet is to pretreat the stain with a commercial spot cleaner, and then soak the garment in color safe bleach and cool water. If that doesn't work, try treating it with dry cleaning solvent.