Microsoft Arc Touch mouse doesn't want to connect in MacOS High Sierra
After a while, the mouse starts working with the next solution, but still, I don't know if it will fail again. 


In System Preferences > Bluetooth at the devices list, delete
mouse and keyboard pairing (maybe keyboard was in conflict, I
don't know)  
Pair the mouse. Turn on, press the Bluetooth button, wait for the device to be identified and connect.  
Then turn on the
keyboard and connect.