Help! My WiFi Isn't Secure

The minute I got my gateway up and running, I've been getting a bunch of notifications from the Xfinity app of phones and computers I don't recognize connecting to my WiFi. I created a password that seems complicated enough. My network is listed as strong and secure, it has the lock logo, so it's not like my password didn't register.

I don't understand how this could be happening. Any suggestions?

Thanks,