Thanks. That helped. When I did that, the ones that didn't show had errors...
The following Android Virtual Devices could not be loaded:
Name: Phone_Android_6_0
Path: /Users/brian.watson/.android/avd/Phone_Android_6_0.avd
Error: Google Nexus 5X no longer exists as a device
Not sure why it no longer exists, but choosing a different device seems to have helped. I can still launch the device from Appium by specifying the name in the desired capabilities, just can't pick it from the dropdown on the Appium UI.