From the Super Mario Wiki
Hanenbow (Japanese エレクトロプランクトン Erekutoropurankuton "Electroplankton") is an unlockable stage in the game Super Smash Bros. Brawl, from the game Electroplankton, unlocked by beating "Event Match 28: Flower Blooms in the Echoes". This stage is a leafy, swamp-like place. The leaves, if attacked, will rock and turn yellow then red. There are Hanenbow that fly around the stage, bouncing off the leaves and can hit players. There is also water at the bottom of the stage which is simply fallen through, unlike other appearances of water. This is one of three stages to have its own icon (a Nanocarp) without having a playable character to go alongside it, the others being Smashville, and PictoChat. Other than this stage, Electroplankton also gets a few Stickers.