Boo Mushroom

"What do we have here...? Looks like a soft, white mushroom, eh? You know, the stuff you're finding is pretty weird... But I guess that's just the universe we live in, right?"

- Lubba

The Boo Mushroom is a power-up featured in Super Mario Galaxy and Super Mario Galaxy 2, which turns Mario into Boo Mario or Luigi into Boo Luigi. It appears as a white mushroom with a Boo's face.

The Boo Mushroom gives Mario the ability to float in midair and move through specific types of barriers by using the Spin command to become temporarily intangible. Negatively, Boo Mario attracts other Boos towards him, who can harm him and revert him back; exposing Mario to a light beam will also get rid of the power-up.

Super Mario Galaxy
The Boo Mushroom appears in the Ghostly Galaxy, the Sand Spiral Galaxy and the Boo's Boneyard Galaxy. Overall, the powerup appears in three missions.

The power-up is introduced in the mission Luigi and the Haunted Mansion in the Ghostly Galaxy, where Mario has to meet and free Luigi by breaking into his sealed room with the help of the Boo abilities, though Luigi is too scared of Mario in his Boo form, so the player has to use a nearby light to turn back to normal.

The Boo Mushroom reappears in the Sand Spiral Galaxy, where Mario has the choice of using either it or the Bee Mushroom to traverse the course. It is featured a last time in the Boo's Boneyard Galaxy, where Boo Mario must race the Spooky Speedster.

Super Mario Galaxy 2
In the mission Haunting the Howling Tower in the Boo Moon Galaxy, Mario or Luigi must use the Boo Mushroom to go through walls and progress to the top of the tower.

Puzzle & Dragons: Super Mario Bros. Edition
Boo Mushrooms are Transformation items in Puzzle & Dragons: Super Mario Bros. Edition. They are used to transform allies such as Red Koopa Paratroopas into Dry Bones or Buzzy Beetles into Bony Beetles, and can be obtained from ? Blocks or dropped by Dark attribute enemies defeated with 10+ combos such as Big Boo, Boo Crew or Boo Trio.

Super Mario Galaxy

 * Trading card bio: "When traveling through the Ghostly Galaxy, Mario needs some extra help to navigate this scary place filled with Boos and Bouldergeists. A Boo Mushroom is just what he needs. When Mario grabs this white mushroom, he transforms into Boo Mario and is able to float, and pass through solid walls for a short time."

Super Mario 3D All-Stars

 * North American website bio: "Float around as not-so-scary Boo Mario, passing through walls and other barriers—at least until he runs into light or water!"

Trivia

 * The card description for this item implies that there are multiple Bouldergeists in the level by using its name in a plural sense, although only one exists in the game.