Problem with videos without a background

[SoCu]

Hi, I'm trying to import an MOV video that doesn't have a background, but it's displaying a black background.

I've tried setting up the chroma key, but it doesn't look right.

I created that video with a green background, and adjusting the chroma key settings helps a little, but no matter how much I fine-tune it, the image has a green tint, or the video's color quality deteriorates.

Why can't MOV videos without a background be used?

Thank you.