The mycelium is actually what the growing of mushrooms is all about. Mycelium is an underground network of cells which can spread over a big surface. The fruit of the mycelium is the mushroom.
In nature the mycelium can form quite large networks. These networks do not mean that the surface with mycelium will automatically be full of mushrooms. The mushrooms grow only where the ideal growing circumstances are gathered.
The mycelium from mushrooms containing psilocybin is white. Strong mycelium also has a somewhat stringy structure (rhizomorph).