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.