Rolling Green Galaxy is accessible from the Fountain observation dome. You need 11 Stars to enter.
This galaxy consists of a number of grass platforms floating in the clouds.
Rolling in the Clouds
Jump onto the ball in front of you. Once on the ball, this level uses a unique control system that doesn't use the Nunchuk. You need to hold the Remote upright, and tilt it to move, like a big joystick. You can still press A to jump.
Follow the path, which mostly involves moving forward. You can squish the Goombas just by rolling into them, or just let them run into you - you don't need to jump on them. The ? Coins reveal trails of Star Bits, if you want.
At the end of the level, climb the flagpole and jump to collect the Star.