Minecraft wiki entry for Mycelium, associated text: It is a common misconception that only mooshrooms can spawn on mycelium. This is not true; it is the mushroom island biome itself that prevents other mobs from spawning there. Friendly and hostile mobs can spawn on mycelium in other biomes just like they do on any other block. Also, mycelium outside of mushroom fields does not spawn mooshrooms.
In real life, mycelium is the part of a fungus that gathers nutrients, comprised of web-like threads known as hyphae. Mycelium can be considered the roots of a fungus, whereas the mushrooms can be considered the fruits.