This stage is Clear Pipe Cruise (6-1). This complete walkthrough will provide you with a 100% video playthrough and a screenshot of where you can find all the green stars.
Game Play Walkthrough Video
Green Star 1
Grab the invincible star as you are passing through the pipes, now you can bust the spiky balls to the left holding the first green star.
Green Star 2
You won’t miss this star, you will simply have to time between the black dusty monsters to grab the second star.
Green Star 3
Head towards the top left once you past the first check point, go through the cloud pipe to start a mini game. Grab the piranha plants and run. The piranha plants will eat the rest of other plants. You will get the final green star at the end of the obstacle course.
Immediately after the check point flag, you will encounter a long clear pipe heading straight up. Instead of going inside the pipe, walk on it to grab the stamp mid way of your long walk.