This stage is Spooky Seasick Wreck (6-2). 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
When you get to the deck with two bullies, push them off the platform to claim your first green star. The best way to kill them is to use skills such as Cat attack or Fire Balls to quickly push them off. You can also jump on them but it may take some exact timing on your part.
Green Star 2
The second star is guarded by some rotating ghosts along your way. Skip to the back and grab the Super Star in the box near by, then use that invincibility power to take out all the ghosts while getting your second green star.
Green Star 3
You will grab the third green star off some narrow decks, carefully walk across to get the final green star.
Go inside the second Warp Box that you come across, and you’ll get transported to a hidden room with the Stamp on the right. You won’t miss the stamp. Wait for the right timing against the ghosts before going for the stamp.