This level only becomes available once you’ve saved Luigi during Luigi and the Haunted Mansion in Ghostly Galaxy. Then, after doing that, you should check in on him near the toad spaceship every once in a while and he will update you on whether he’s found any secret hidden stars. Shortly after telling you about a hidden star in Good Egg Galaxy, you’ll receive some mail from the Mailtoad that’s sent by Luigi. He has found a star but is now trapped and needs your help!
Trapped in Good Egg Galaxy
From the photo that Mailtoad gives you, you’ll be able to recognise the capsule-planet found in Good Egg Galaxy. You’ll need to head over the Terrace dome and go to the Dino Piranha level in Good Egg Galaxy.
Luigi on the Roof
Luigi can be found on the roof of the little toad house on the starting planet. You can either use the orange warp pipe on the otherside of the planet to reach Luigi, or you can use a faster shortcut. If you triple jump or backflip onto the sloping section of the roof then wall jump off of that, you can spin midair and adjust your positioning so that you land on the roof where Luigi is. When you speak to him, he will give you a Power Star!