You'll need 90 gems to unlock this level.
Captain Toad finds himself in an area with many clear pipes, Piranha Plants, and Fire Piranhas. Go through the first few sets of clear pipes until you reach an area with a nearby cloud. Stand in front of the pipe near the cloud to lure the Fire Piranhas into breathing fire into the pipe. This will destroy the spike balls inside. Go through the pipe for a Gold Mushroom. Up ahead is a large pipe with a pickax, some spike balls, and a warp box at one end. If you like, grab the pickax so you can destroy the spike balls and get more coins. Warp to the next area and hit the POW Block by touching the GamePad. You'll get a Mushroom from the item block above. There's a ramp leading up. Drop from it to get rid of the Piranha Plant in front of a pipe so you can get more coins and reach the top without dealing with more Piranha Plants. At the top is a chain of Fuzzies moving through the clear pipes. Use the clear pipes to carefully cross the gap so you can reach the clear pipe cannon.
The cannon will take you to an area with a few more Fuzzies and some clouds. Use the pipes to reach the moving clouds above, then go through the warp box. This will warp you higher up. There's a Fire Piranha in front of a clear pipe. Lure it into breathing fire into the pipe to clear out the spike balls, then go through the pipe for a 1-up. Slide down the hill with the many Piranha Plants down to a complex clear pipe network with Fuzzies. There's a Key Coin at the base. Pick it up, then go through the pipe with good timing. The other four Key Coins are at the top parts of the pipe network. With careful timing, move through the loops, going in the same direction the Fuzzies are. Once you grab all of the Key Coins, the Warp Box at the top will be unlocked and you can warp to the Green Star.
Special Challenge: Collect the Gold Mushroom.