How do I find out the address of [Settings>Display->Cast] devices?
You can't easily identify its MAC address as its not connected to any AP.

Wireless display (WiDi) does not require any Wi-Fi Access Point or router or hotspot for operation. It uses WiFi Direct® (or Wi-Fi CERTIFIED™ Miracast®) technology, for wireless connection to WiDi compliant receiver.

Having said that, it therefore implies that by your device identifying [TV] Samsung 6 Series (50) in Settings→ Display →Cast is not connected to any AP, but it just being detected by your device as a "nearby device" and does not even reveal its MAC/IP explicitly even if you make the connection.



However the easiest way to identify its MAC/IP is allowing it to connect to any of available wireless AP:


Go to your TV's menu and choose Network → Network Settings
Choose Wireless for your connection type
Choose to connect to the  SSID
At this point, the Smart TVs will show you a screen with the Wireless MAC address.
If not, go back to the Menu and choose Network → Network Status. 


Alternatively check on your device's list of connected devices and get the IP.



Note: I have even observed that my Smart TV (Samsung) shows in the list of Available Wireless Display devices on my tablet, even before connection.