How to I allow my variable to be used and have my Arduino execute automatically?

I calculated an updating theta value on my Raspberry Pi, I'm sending that over to another Raspberry Pi because of the distance my project requires via MQTT, and what I want to do is to be able to send that variable over to my Arduino to turn my stepper motor to that angle via a USB connection.

I have some simple small code on my Arduino already that takes a dummy variable to change the angle already and I'm almost done with my MQTT nodes.

What node(s) should I use to take in my variable from my Raspberry Pi, have it to be used by the Arduino, and then execute the Arduino code automatically?

This is the basic structure I have so far:

timestamp --

exec (My Python Script, while the command is running, spawn mode is enabled) --

change node (removes the string "Theta" that was shown in the original code (("Theta: " , theta))

debug

The serial node