From the Super Mario Wiki
Polterguys, originally called Phantom Guys, are ghost Shy Guys found in Mario vs. Donkey Kong 2: March of the Minis. They are very similar to the Shy Guy Ghosts from Mario vs. Donkey Kong. When a Mini Mario presses a button the same color as the ghosts, the Polterguys will turn into blocks, allowing the Mini Marios to pass. They appear in the Spooky Attic stage. Polterguy comes from the words "poltergeist" and "Shy Guy".