Mycelium

Mycelium is a new block type added in the 1.9 pre-release, found in Mushroom Island Biomes. It acts like dirt and when harvested with a shovel will yield one dirt block.

Mycelium slowly spreads to other blocks much like Grass. However Mycelium cannot spread on grass and vice versa. Saplings and Flowers can not be placed on Mycelium, but Mycelium can spread beneath them, which will make them "pop" out.

Minecraft provides some effects for the top of Mycelium blocks which look like tiny spores being released constantly from the surface. Mooshrooms will spawn on it.

Mycelium is the only block on which mushrooms can stay when an adjacent block is updated despite a light level that would otherwise be too high.