With the right USB OTG cables, I was able to connect my Nexus 5X to a beaglebone black and a beaglebone green. I had to try several cables before the beaglebone would power up; I suspect it's the USB-C adapter that's the most problematic. This is the USB-C OTG cable that worked.
Once the board booted, I got a notification on my phone about the beaglebone USB storage device becoming available. But I wanted to send data back and forth between an android app and a beaglebone process, so the network interface was the important thing to me.
When I connect the beaglebone to my PC, it shows up as a USB ethernet adapter, and I can talk to it at 192.168.7.2. I downloaded an android app called "Terminal Emulator", and when I ran "ifconfig" I could see that I had an eth0 device with IP 192.168.7.1. But I couldn't connect to it.
But if I turn on airplane mode, oddly, I can connect just fine by putting "192.168.7.2" in the address bar of the browser. I haven't figured out yet whether it's possible to have LTE or Wifi on and still reach the beaglebone; perhaps it's just something to do with the IP addresses used by the Wifi or beaglebone.