You're running the SlingStudio Capture app on an Apple iPhone XS, Apple iPhone XS Max, or an Apple iPhone XR, using the iPhone as a video source. When you change the orientation of the iPhone from portrait to landscape, you may see a green screen.
This can happen with this combination of Apple iPhone models and the SlingStudio Capture app.
If this happens, rotate the Apple iPhone 180 degrees to clear the green screen. After doing that, the Capture app should work properly again.