From the Super Mario Wiki
The Ghost Shroom is an item in Super Paper Mario. It summons a Ghost Shroom to damage enemies. When used, the player must draw a circle around a picture of a Mushroom with the Wii Remote to make it become larger, similar to the Ruby Star's Action Command in Paper Mario: The Thousand-Year Door. The larger it is, the more likely it will hit enemies when it becomes a Ghost Shroom. It then follows the player for a set amount of time. Players can buy it at Notso's shop in Flopside.
Any recipe that requires a Poison Shroom can be cooked using a Ghost Shroom instead. This is useful if Poison Shrooms are in short supply.
The in-game description of the Ghost Shroom is "Summons a Ghost Shroom to snack on your enemies".