RT-AC1900; How to block device w/o MAC or IP address?

While looking at network traffic I found a device on my network I do not recognize (Google Nexus 4). Small family here and no one has that device. It shows up in the "Traffic Analyzer - Statistic" client list (see screenshot) but not not in the network clients list so I cannot see an IP or MAC Address for the device.

Is there another way to identify the device?

I want to block the device, how can I do that?

Thanks!

Ken