I'm not entirely sure why this happens, it happens intermittently for me. The last attempt I took was Delete the controller, Turn off Bluetooth, Restart, Reconnect Controller- but while I was reconnecting there was a phantom controller that popped up that showed the original controller connected, but I could not disconnect it through the Mac, I turned the controller off and the phantom controller popped back down to the unknown devices and upon entering bluetooth mode on my ps4 controller the phantom controller was identified as a ps4 controller and I was able to connect and use the controller again to play games. I hope this helps!