You can remove mud by first letting it dry thoroughly and brushing off as much as possible. Then you can rub the stain with Murphy's Oil Soap, leave for 15 minutes, and rinse with cold water. Or, instead of the Murphy's Oil Soap, you could use a solution of 1 tablespoon borax in a cup of water. If the spot persists, use an enzyme presoak, or soak the item in a gallon of water to which you've added a cup of ammonia (or use bleach if the items are white).
Another option to try is to slice a raw potato in half and rub the mud stain. Soak the item in cool water for a while, then launder as usual.