From the Super Mario Wiki
World Mushroom-B is the second secret level of World Mushroom in New Super Mario Bros. 2. This level is an autoscroll level composed of many Beanstalks and Koopa Troopas. To unlock this level, the player needs to locate the secret exit in World Mushroom-.
First off, Mario begins on a cliff in front of Beanstalks, mushroom platforms and Koopa Troopas. Moving forward, there is a green pipe which takes the player to the main area. As Mario comes out from the pipe, the level begins autoscrolling. A Chain Chomp can be seen on the right. Above, there are Brick Blocks, ? Blocks and a red POW Block. The Brick Blocks contain Beanstalks and the ? Block on the right contains a Super Leaf. Climbing up the Beanstalk, there are several coin rings spinning around. Even more Beanstalks appear from blocks where there are several Koopa Paratroopas. A Chain Chomp can then be seen on a green mushroom platform. Going past it, there are several hidden blocks containing 1-Up Mushrooms on the right. Going further up is the Checkpoint Flag. More Beanstalks and a Super Leaf can be found. This is followed by circling Paratroopas and a Red Ring. Just after that, several Koopas and a Gold Ring can be found along with more blocks containing Beanstalks. Another Paratroopa ring can be found enclosing the third Star Coin. Further up are more Beanstalks and Chain Chomps. After that, a green pipe leading out of the area can be found. Moving right, a lone red Paratroopa can be seen, followed by the Goal Pole. Completing this level will unlock World Mushroom-.