How does a mud flow form?

liquid.

Mud that lay on a hill and when heavy rain comes the mud flows down the slope to create aMUDSLIDE.

Liquid mud drips from a more elevated point onto one spot and hardens. this is repeated many times until a mound or stalagmite is formed.

It's very simple: soil + water = mud

Lava cools to become a crust of stone with many small air bubbles, often called lavarock