I bought a Chromecast yesterday and came to the same painful conclusion that casting directly from MyCloud is not possible on IOS and that most suggestions in the thread above are limited or not functioning that well.
I bought the Infuse Pro 5 app from the appstore for EUR 9. It was very easy to synch with various cloud services, WD my Cloud as well. It can Chrome Cast all video files, no hassle at all. Besides that, various subtitles formats are supported as well.
When I early on mentioned to you that any app with ability to access My Cloud AND had casting ability could cast to Chromecast, and I suggested using the File Browser app) you totally and impolitely blew me off because the app costs a lousy six dollars.
Mow you thank the latest poster for suggesting Infuse app which costs more and does less. What’s with that?