Hi,
Add all the aliens to a group. When the player dies, send a signal to the group. When the alien gets the signal, make it die.
So something like:
in the alien script _ready()
function:
add_to_group("alien")
Add a function for the alien to react to the death of the player:
func player_dead() -> void:
# make the alien gloat ;)
queue_free()
Then in your player script, when the player dies:
get_tree().call_group("alien", "player_dead")