From the Super Mario Wiki
Retrieved from "http://www.mariowiki.com/index.php?title=Blue_Mouse&oldid=1892293"
Blue Mice are the ghosts of mice found in the game Luigi's Mansion. They run along the floors of most of the hallways and cause five HP of damage if Luigi touches them. They are very common and have no HP value; they are simply sucked up instantly.
The Blue Mice are usually found in the dark halls and occasionally hidden places.