From Halopedia, the Halo wiki
Grunt Mob are a group type unit employed by the Banished.
Grunt Mobs are a variation of the standard Grunt Squad employed by the Banished. Upon deployed, they rush into battle.
Grunt Mobs are assigned to a group of 3 Grunt Squads that appear in the Blitz multiplayer mode of Halo Wars 2 as a playable card for all Banished leaders. Unlike it's multiplayer counterpart the Grunt Squad, the Grunt Mobs come in massive numbers and also with a extra Grunt to each squad instead of a Brute.
The Grunt Mobs are classed as core infantry, and performs well against infantry, vehicles and aircraft.