how to send data to Adafruit Bluefruit Feather (32u4) over bluetooth?
The “UART” being referred to is the Nordic UART GATT Service. This is not a serial port or USB connection. It is a high level BLE protocol that simulates a serial port via the Bluetooth connection. 

In order to connect to the device from your Mac you’ll need to write some C or Obj-C. I’ve done this for Windows, but not a Mac, so I’m afraid I can’t give you detailed instructions. However, the source code for the iOS Adafruit Bluefruit LE Connect app you’re using is open source and should be a good starting point. 

Be warned of a few things though. 


You’re going to have to learn a bit about the BLE protocols and standards. It’s not exactly for the faint of heart. 
Apple throttles the speed/throughput of BLE for their iOS devices (as does Android) to protect battery life. I don’t know if they apply the same throttling to their laptops. You may not be able to achieve a satisfying data rate if you’re looking to create a high throughput “data pipe”.  With Win10 and the Bluefruit LE I was able to achieve 40 bytes every 30ms. You’ll likely be limited to 20 every ~30ms. 

If you’re looking to just send some occasional text (maybe AT style) commands to the device via BLE though, you won’t come close to those limitations.