From the Super Mario Wiki
Honeycomb Walls are special hexagonal walls made of honey that only Bee Mario can climb on in Super Mario Galaxy and Super Mario Galaxy 2. They have their own gravity, as Mario can fly with his feet facing the wall and even Ground Pound onto the wall.
In Super Mario Galaxy, Honeycomb Walls are featured prominently in four different galaxies: Honeyhive Galaxy, Honeyclimb Galaxy, Gold Leaf Galaxy and Sand Spiral Galaxy; however, only one lone honeycomb wall panel appears in Sand Spiral Galaxy. When Bee Mario climbs on these vertical walls, he sticks to the honey on it. Bee Mario must fly to reach different parts of the wall. The Honeyclimb Galaxy is made up entirely of Honeycomb Walls on what looks like a flat hive. Some Honeycomb Walls can contain moving groups of honey, Mandibugs, and even meteors falling past them.
Honeycomb Walls reappear in Super Mario Galaxy 2, only in the Honeyhop Galaxy.