From the Super Mario Wiki
The Emerald Star is the second Crystal Star and was found in The Great Tree in Paper Mario: The Thousand-Year Door. It was stolen by Lord Crump, but given to Mario by the Puni Elder after driving the X-Nauts out of the tree.
This Crystal Star will teach Mario the move Clock Out, which costs two SP. To use this move, the player must repeatedly press the button that appears on the opponents side of the field. If done correctly, enemies will be immobilized for a few turns. However, the buttons will change at random intervals, and the player may end up freezing Mario and his partner by accident. The audience may be also frozen if it is neither on Mario and his partners' side or the enemies' side. Additionally, the Shy Guys in the audience can protect the enemies from this attack.