From the Super Mario Wiki
Foos are white cloud-like enemies that appear in New Super Mario Bros. Wii, New Super Mario Bros. U, and New Super Luigi U. They produce fog that can obscure the player's view. Foos can be defeated with any kind of attack. Their name comes from the Japanese onomatopoeia for blowing sounds.
New Super Mario Bros. Wii
New Super Mario Bros. Wii marks Foos' debut appearance. They act much like a Fwoosh from Super Mario 64 and its remake, though Foo's wind only produces fog, and does not blow Mario away. They appear only in World 7-5.
New Super Mario Bros. U
New Super Luigi U
Mario Party 10
These gusty goons play a role in the Mario Party 10 Mini-Game; Foo Me Once. In this game, players must remember 7 platforms with A, 1, and 2 marked on them, which will then be covered by the fog from the Foos. There is only one platform per jump, meaning there is no room for mistakes.