From the Super Mario Wiki, the Mario encyclopedia
The Goombas attached to floating balloons appear in New Super Mario Bros. U. They are basic enemies attached to balloons, acting similarly to the Para-Goombas of Super Mario World except floating from the opposite direction.
Balloon Goombas only appear in the Sparkling Waters stage Skyward Stalk, where six of them are found. 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.