Boudin is a sausage from south Louisiana made with a rice stuffing mixed with ground pork or crawfish (yum). The Mousse, I assume, is in reference to his species (mouse) since I have never had boudin for dessert.
If you ever go to a hardware store and pick up some "Backer Rod" the French translation on the package is "boudin mousse." I always thought that was a good name for a Mouse bandit. ^_^