Garmin Express should be used exclusively on the Mac. Garmin Express handles all the communication with Garmin fitness ANT devices. The Garmin ANT Agent application should be uninstalled from the Mac. Please see the FAQ: How do I uninstall ANT Agent?
If you simply want to Quit the ANT Agent, click the Mickey Mouse looking icon at the very top of the screen in the Menu bar and choose Quit or Exit ANT Agent.
