Erase that graffiti!
From the Super Mario Wiki
Mario and co. must head to Fahr Outpost to find Swob, who will tell Mario of some graffiti on the fiftieth floor of the Pit of 100 Trials. Mario must head there and use Admiral Bobbery's ability to destroy the graffiti. Mario must then return to Swob, and he will reward Mario with a Snow Bunny, ending the mission.