This stage is Shadow-Play Alley (2-3). This complete walkthrough will provide you with a 100% video playthrough and a screenshot of where you can find all the green stars. We will also note a secret 1 UP.
Game Play Walkthrough Video
Green Star 1
The first green star casts a shadow in the background. Walk towards the TV screen (You) to grab it.
Green Star 2
Break through the purple wall when you get to it. Go inside the purple box. The green star casts a shadow and you can get it by going around the stone staircase.
Green Star 3
When you get to the red door, do not open and past it. Instead walk towards the right. Break the wooden Bowser and save Toad. The grateful Toad will then reward you with the final green star.
While Mario is in Shadow form in the back ground. Break the blocks under the Piranha plant. You can also note whenever you see the shadow of the stamp. You will have to use Cat Mario’s claw attack to get this stage’s stamp.