mycelium

As a noun mycelium

is the vegetative part of any fungus, consisting of a mass of branching, threadlike hyphae, often underground.

As an adjective mycelial is

of or pertaining to the mycelium.

As nouns the difference between mycelium and sclerotium

is that mycelium is the vegetative part of any fungus, consisting of a mass of branching, threadlike hyphae, often underground while sclerotium is a compact mass of hardened mycelium stored with reserve food material that, in some higher fungi such as ergot, becomes detached and remains dormant until a favourable opportunity for growth occurs.