I want to clarify something. So basically, what you want is, when your Monster spotted the player, you want the monster to chase the player?

My suggestion is to use signals and groups.

func ready():
connect("body_entered", chase) #the syntax may differ depends on the godot version

func chase(body):
if body.is_in_group("players"):
player_path = body.get_path()

If you have any more questions please ask. I just started leaning godot last year so there is a chance that i get things wrong but this is what I learned and it works for me.
I just wanted to connect to people.