When a Pokémon with Mycelium Might uses a status move, it always moves last within its priority bracket, and the effects of all Pokémon's ignorable Abilities are ignored for the execution of that move. (Ignorable Abilities are most Abilities that could potentially negatively affect the success, damage, or effects of a move if possessed by the target of a move or its allies.)
For example, a Pokémon with Mycelium Might that uses Screech will move last in its priority bracket, but can affect opponents that have Big Pecks, Clear Body, or Soundproof.
Outside of battle
Mycelium Might has no effect outside of battle.