A Comet Piece.
Comet Pieces are items that appear in Paper Mario: Sticker Star. They come from the legendary Sticker Comet.
Comet Pieces are large star pieces that changes color each and every time. After Mario gets the Comet Piece and returns to its original location, it is now clear in color with a yellow background.
Comet Pieces main purpose is to end the level that Mario is playing on (except for levels that have the World's boss at the end. The Chapter itself ends with Mario getting the Royal Sticker). With levels with Secret Exits, there is two Comet Pieces located in that level. Only in the level Wiggler's Tree House is where three Comet Pieces could be found.