Hi David,
Thanks for posting your question here in the Community. I understand that you wanted to remove the Arc mouse it the bluetooth settings. That issue happens due to driver issues, registry remnants or stuck Bluetooth services. To fully remove it, you can do these.
First, via Airplane Mode>Press Windows+A > enable Airplane Mode for 15 seconds>then disable it. Go back to Settings>Bluetooth and devices>and try removing the mouse again.
If that don't work, try removing it Device Manager>Press Windows+X and click Device manager>click View>Show hidden devices>expand Bluetooth and Human Interface Devices>right click the arc mouse and choose Uninstall Device>then restart your PC.
Let me know if it works for you.
Cheers,
Res