After effects - how to hide dropping animation over white borders?

Hi there!

I am putting my compositions together in After Effects and I am trying to make a video with a white border around each composition. On multiple compositions I have animations dropping down from above, and I don't want them to drop over the white border in the final video. I thought that since they are separate compositions added on top of a white background in the end, the objects going over the border of the composition wouldn't be visible... but they are! How do I hide them so it looks like they are actually dropping from the sky?

Thank you for your help!