Clean Mouse is a skill in the Wind Master skill tree. For each skill point allocated in this skill, a bubble will spawn at the place of a mouse's death. The bubble will appear to go from the bottom of the map and progressively float toward the top of the map. The bubble's behavior is identical to the balloon's; it travels at the same speed and pops when more than three mice stands on top of it.
Bubbles appear quite often in the Transformice cartoon series.