New Super Mario Bros. U: Acorn Plains-4 -- Mushroom Heights

It's a mushroom platforming level. Some of the mushrooms move. The clouds overhead conceal coins. Pink Baby Yoshi is a great help here. There are strange-looking Goombas walking on the mushrooms, but they don't walk off. Use Baby Yoshi to reach the first Star Coin over the blue mushroom. The midpoint is on a different blue mushroom. The second Star Coin is under a blue mushroom, but it's over the void. Use Baby Yoshi to get it, repeatedly hitting [ZR], until you're over a mushroom to stand on. Go down the green pipe between the four pink mushrooms that expand and contract. It will take you an area with moving mushrooms and some Goombas. Kill the Goombas first, then activate the Green Coin Ring. This is a new ring, that makes green coins appear in triplets. Get all three before they soon disappear to make another triplet appear. All told, five sets of three green coins will appear. Collect all fifteen green coins to get a powerup. Return to the main level and you'll find the third Star Coin under a tilting green mushroom between two giant Piranhas. Baby Yoshi can't eat giant enemies. It may take a few attempts to get it. After that is the flagpole. This opens the path to the castle.