Mycelium

Mycelium is a new block type found in Mushroom Biomes. It acts like dirt, and when harvested with a shovel will yield one dirt block. It slowly spreads to bare dirt blocks. Mycelium acts exactly like grass even to the point of growing faster when light is near which is contrary to the habits of real fungi. It can grow out of its biome if provided a way; mycelium and grass do not replace one another. Saplings and Flowers can not be placed on Mycelium, but Mycelium can spread beneath them, which causes them to drop as items from the spot. The tops of Mycelium blocks have a particle effect which look like tiny spores being released constantly from the surface. Mooshrooms will spawn on it. Mycelium is also the only block on which mushrooms can stay when an adjacent block is updated despite a light level that is too high for the Mushroom to remain. Unlike dirt or grass blocks, Mycelium cannot be tilled with a Hoe. Monsters cannot spawn on Mycelium.

Trivia

 * The player can only obtain Mycelium in their inventory via an inventory editor or multi-player commands as an OP.