Ice Snake Block
From the Super Mario Wiki
Ice Snake Blocks are a type of Snake Block in New Super Mario Bros. Wii. They appear in World 3's castle. They are very slippery, and Mario, Luigi, Blue Toad and Yellow Toad should be careful not to slip off of it, because there is a bottomless pit under the Ice Snake Block. Only two are found in the stage - the first one that leads to the checkpoint, and the second one that leads to Lemmy Koopa.