From the Super Mario Wiki
Crooked Cavern, or Acorn Plains-2, is the second course of Acorn Plains, and subsequently, the second course of New Super Luigi U. It's unlocked from Waddlewing Warning!, and its own completion unlocks Flame-Gear Tower and the first Toad House.
The first area is a small overworld field where three pipes are found, the left and right ones are red and contain Piranha Plants, while the middle one is green and leads to the cave. This cave is made of many moving crystals and red pipes with Piranha Plants. At the end, there is a Warp Pipe above a pit to the third and final area.
Right after the third Star Coin, ground pound the spot where the Arrow Sign is and follow the signs to the secret exit.