Mycelium grows from the bottom up, branches splitting and splitting again until the network fills the frame, then gently fades and regrows. Every tip glows while it's actively growing, then goes quiet just like real mycelium.
The mycelium branching is hyphal growth modeled with something called an L-system - a mathematical grammar that describes how branching organisms grow just like trees, lungs, rivers, lightning - they are all L-systems.