The green flashing light under the Magic Mouse flashes when it is trying to connect to a device. If your Magic Mouse is continually flashing and not connecting, do not worry it may not be broken. You will need to reconnect the device.
How-to Reconnect the Magic Mouse
1. Click on the Bluetooth icon on the top toolbar and go to ‘Open Bluetooth Preferences…’
2. Hover your mouse over your previously paired Magic Mouse and hit the ‘X’
3. ‘Remove’ the device
5. The Magic Mouse will now be paired to your computer.