Many fungi grow a mass of hyphae usually hidden within the material on which it is growing. what is this mass of hyphae called

Mycelium.

Explanation:

Mycelium is a mass of thread-like hyphae that can grow on a surface, decaying material, soil or on a living tissue. It is the vegetative part of a fungus through which a fungus absorbs nutrient from the environment. They are good sources of food for soil invertebrates. They also play a major role in decomposition of plant material.