Are you sure it's a Bluetooth mouse? The dongle might be a 2.4 GHz wireless transmitter, rather than Bluetooth. The only reason for a dongle would be to allow a computer that doesn't have Bluetooth built-in to be able to use it--the dongle would therefore be a USB Bluetooth adapter.
Do you see the Bluetooth symbol anywhere on the mouse or the packaging? Apologies if this is a dumb question, but for those without lots of experience, it can be easy to mistake a wireless mouse for a Bluetooth mouse.