This stage is Mystery House Throwdown (6-Mystery). 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
Toss a baseball into the star.
Green Star 2
Jump first so that the star is on the bottom site. Then jump and toss the baseball towards the green star.
Green Star 3
Launch yourself off the Jump Panel and toss a baseball at the P switch.
Green Star 4
Hit the POW Block with a baseball to destroy all the enemies to reveal the green star.
Green Star 5
Three Thwomps guard three switch panels. Use the timing gap to throw baseballs at the P switch.