What is the huge cloud sliding on the volcano called?

The huge sliding cloud on the volcano is called pyroclastic flow, also known as tephra.

Explanation:

The volcanoes, without exceptions, have eruptions. The eruptions differ though, and it is not just the stereotypical eruption with lava flows and gas propelled high into the atmosphere, but there are many other elements. While pretty much all of the elements of a volcanic eruption are dangerous, some more some less, the most dangerous one comes in the form of a sliding cloud.

This sliding cloud, coming down the sides of the volcano and continuing in the valley below it is known as pyroclastic flow or tephra. It is a very dense cloud of very hot gasses, and on top of it moves very fast, so there isn't escape from it really if someone is on its path. The pyroclastic flow has such a devastating effect that it literary manages to scorch and level down pretty much everything on its path, killing every living organism in the process as well.