Add a connection to erase the mob when out of screen

Enemy.gd

@@ -9,3 +9,6 @@ var mob_types = ["walk", "swim", "fly"]
 
 func _ready():
 	$AnimatedSprite.animation = mob_types[randi() % mob_types.size()]
+
+func _on_VisibilityNotifier2D_screen_exited():
+	queue_free()

Enemy.tscn

@@ -48,3 +48,4 @@ rotation = 1.5708
 shape = SubResource( 2 )
 
 [node name="VisibilityNotifier2D" type="VisibilityNotifier2D" parent="."]
+[connection signal="screen_exited" from="VisibilityNotifier2D" to="." method="_on_VisibilityNotifier2D_screen_exited"]