Frame difference mask
The effect is known as a "difference" key. It's also known as a "not very good" key, because unless your footage is shot on a rock-solid locked off tripod and you're shooting uncompressed 4:4:4, it works a lot less well than you'd think.

After effects has a Colour Difference Key effect, as does Premiere, and this forum post explains how to do it in Apple Motion - though the principle will work in most compositing applications.

And of course, if you're after free as in speech as well as in beer Blender has a difference key node too.