Every category you might reach for is wrong, including “colony”.
Physarum is one cell. Somewhere in the course of growing it stopped bothering to build walls between its nuclei, so what spreads across a log — or a petri dish, or a scale model of the Kantō region — is a single continuous bag of cytoplasm with millions of nuclei adrift in it, pulsing.
That pulsing is how it thinks, to the extent that word survives contact with it. Cytoplasm surges back and forth through the veins, faster toward the parts of itself that are finding food and slower toward the parts that are not, and tubes that carry more traffic get thicker while tubes that carry less waste away. Nothing decides this. The rule is local and the network is what the rule leaves behind.
Give it a maze, and the dead ends drain and vanish. Give it oat flakes arranged like the towns around Tokyo, and what grows is close enough to the rail network to be uncomfortable. Give it something bitter to cross, over and over, and it stops flinching — and it can still remember not to flinch after being dried out and brought back.
One rule about tube diameter, running everywhere at once. That is the entire apparatus.