That moss-looking stuff is an evil weed persistent beyond your wildest nightmares. Its roots go a half inch deep, form a tight mat, and embed themselves in the upper roots of the tree. You could play football or race elephants on that surface.
Moss has "feet" and connects to the ground, and to the tree if you let it, but doesn't have roots. It looks good, cools pot roots, and modestly conserves moisture.