Videos edited through iMovies Flickers and Flashes

I removed background from iPhone video and made it green then changed background in iMovie but the videos object flickers and flashes how do I fix this

I used YouCam app to make video background green and when I use this video in iMovie to change green background into another background the video flashes and flickers over original video and objects in it. I use the blue/ green screen function in iMovie to replace background

[Re-Titled by Moderator]