This happens to me occasionally on an Android device. I commented on it previously in the following post:
https://community.wd.com/t/using-mycloud-to-stream-to-chromecast/172050/13
In my case, when it happens the solution is to exit out of the WD mobile app, then turn off WiFi on my mobile device, wait a few minutes, then turn WiFi back on, then relaunch the WD mobile app at which point the Chromecast icon is available.
