From the Super Mario Wiki
The Silver Gearmo is a minor character in Super Mario Galaxy 2. Its body and gear are silver, like the Gold Gearmo's are gold. Like its golden-clad counterpart, it wants the player to lead an enemy into its small pen (indicated by four pegs sticking out of the ground, making a small square) once it invites them into Space Storm Galaxy. However, instead of a Goomba, it wants a red Topman.
If Mario or Luigi manages to get a Topman to the pen, the player is rewarded with a Power Star, as promised. The Silver Gearmo will then appear on Starship Mario, sipping tea with the Gold Gearmo. When talked to, it thanks the character for his efforts and gives him a 1-Up Mushroom. After that, all it says is "I love how those red guys spin!"