I believe that when your Android phone is in WiFi Hotspot mode, it is not connected to the wifi network it creates. Therefore you won't be able to select/connect to the chromecast since the phone itself is not on the same network as the chromecast.
Posts made by xbiggyl
RE: Streaming Over Android Wi-Fi Hotspot
Route Audio to PC or Android
Your app is the top one in it's category IMO. Kudos!
The only missing feature is to have the option of choosing the device that outputs the audio while playing the video on TV
In other words, it is missing the option of playing a video on Chromcast, while the audio is outputted from the PC or Smartphone.
This feature is available on LocalCast and works flawlessly. It's called Route Audio.
I use Videostream 90% of the time. However, every time I want to play a video on my TV and have the audio on the smartphone, I am forced to use LocalCast.
On LocalCast, they added the option to adjust the delay of the audio in case it turned out to be out of sync. So with a couple taps the audio is in sync and I can listen on my headphones.
I searched around and found out that many people have asked about this feature in forums/reddit and how they can stream video and have audio come out of their PC. LocalCast can only do it on smartphones, yet it is the app that everyone is recommending for the inquirers to do the same but on Android.