If I remember correctly, wood stain remover is the ideal product for mustard. Have heard french’s yellow mustard is supposedly the worst to get out out of the mustards available. Have also heard that placing a UV light over a mustard stain for at least 6 -12 hrs may possibly lighten it, although I don’t know what it would do to the carpet fibers that don’t have mustard on them. Could alter them as well, and frankly, who has 6 hrs to sit there and see if/when this works?