“Goblin” are a type of Vex.
These are swarm type enemies for the Vex. They focus on simply overwhelming the Guardian with their sheer numbers.
If the head is shot off, the automaton will keep moving. If anything, it increases its speed and frequency of attack. It has amazing accuracy for a "blind" creature.
They can also crouch. If they do, their vulnerable point is covered up. It is very easy to shoot of their head while they are doing this. The Goblin is also more vulnerable to melee attacks.