This is an underground level. Lots of Buzzy Beetles and Swoopers. The secret exit that leads to the Green Switch Palace can be reached by entering the green pipe that's after the green block among the rising yellow stones. You need to be able to fly, either with Yoshi or a cape. The four yellow blocks that form a staircase lead to a chamber that holds the key. The last block has a vine in it, directly above is the entrance to the chamber. Flying in just with the cape can be tricky since the entrance is narrow, so it's better to let the vine grow and grab it.