From the Super Mario Wiki
Balloon Goombas are, as their name implies, Goombas attached to floating balloons. They first appear in New Super Mario Bros. U.
New Super Mario Bros. U
Balloon Goombas' only appearance is in the Sparkling Waters level Skyward Stalk. They float straight upwards from the bottom of the screen on a balloon. When its balloon pops, it will begin walking like a normal Goomba. Mario can jump off the balloons for extra height. There is a total of 6 in the game making them rare.