From the Super Mario Wiki
Seesaw Shrooms are a type of a Mushroom Platform found in New Super Mario Bros. U, New Super Luigi U, and the Nintendo Switch port, New Super Mario Bros. U Deluxe. They are normally orange with a circle in the middle but have blue variants in ice levels as well. They appear in different sizes, and can be almost as large as a full screen.
Seesaw Shrooms tilt slightly from side to side when idle, but balance under the weight of players when they stand on it. Enemies and objects do not affect the balancing of the platform.
Seesaw Shrooms first appear in New Super Mario Bros. U and New Super Luigi U, appearing in a level named after it Seesaw Shrooms in the former game, while in the latter a blue variant appears in the level Cooligan Shrooms.