Fog Mario is a form Mario can obtain when he gets a Fog Flower. The item is rare, and it could be found in Super Mario Galaxy 3. The form lasts for the rest of the level, but Mario cannot take it along on the map.
Fog Mario has the ability to become intangible, and could float through bars of any kind. He is inpervious to anything. The only thing that could effect him is water and strong winds. This form can allow Mario to bury enemies deep in fog and make the enemy stationary for 30 seconds. But it cannot defeat enemies.