A device that I don't own was logged in on my guest network!

I recently bought a Netgear Nighthawk XR700. I have the thing all setup and running and today I was looking at the system logs and notice that it read that my macbook was granted "access control". This is odd because no one had used the macbook all day.  So, I deceided to investigate the devices that where connected and noticed an "unnamed device" connected with the same mac address that had been listed for what the system report stated was my macbook. I then checked to see if the macbook was on the list of devices and it was with a different mac address altogether.  

Can someone tell me how a unknown device (person) managed to connect to what is supposed to be a secure network and grant themselves access control?

How do I keep this from happenening again?

Why isn't there a secure login?

How can I device be "offline" and still be "online"?

Also, can't I delete a device off of my network if it's connected? If it's unwanted why can't I cut their access?

P.S. I used the nighthak app to report a lan disconnect problem that I got yesterday after connecting the router and playing on PS4 (didn't happen on my ASUS ROG or my google fiber router but happened an hour after setting up the XR700). I had a weird interaction with "Elsa P" through support on the app. The part of the interaction that I thought was suspicious was that before she could proceed she needed to know if I was using the app to connect to support. Why was this such an important bit of info for her and why did she need all the information for the router when the thing was registered automatically when I logged into the app and obviously available to her as well?