Phone listed in Device Manager isn't mine
Follow the instructions below. I suggest you should change your password just in case. 


Use your phone's browser to go to myaccount.google.com. 
In the "Sign-in &amp; security" section, touch Device activity &amp; notification.
In the "Recently used devices" section, touch Review devices. 
Touch the device you want to remove > Remove.


Source: https://support.google.com/accounts/answer/6264236?hl=en