From the Super Mario Wiki
The Hedron is an obstacle in Super Paper Mario encountered in the Outer Limits. Its name comes from the geometry suffix -hedron. The Hedron is an indestructible, rotating, blue rectangle with a yellow outline. If Mario is hit by it, he takes damage. Hedrons are often encountered in groups and must be avoided with quick, evasive, maneuvers. Flipping to 3D does not help, either, as the Hedron exists in the third dimension as well.