Sounds like you're using a translator.  If you are, use proper punctuation before translating it because your whole post is incoherent.

You have footage, you can mask out the background, but what do you mean background under transparent element?  Do you have transparent objects in the scene with the original background behind it?  If so...tough luck, should have gotten some green poster boards from your local office supply chain.

Otherwise, please elaborate with some screenshots.

Well, like I said, too bad.  These things should have been addressed prior to animating/filming.  It is why people use greenscreens.  If possible, try isolating the transparent bricks using individual masks and tweak them until they fit your need.  For example, adjust their transparency separate from the rest of the scene, key out the background, or change selective change certain colors to make it blend with the new background.

Yep, it's as easy as using a couple of copies of the footage layer and masking the appropriate areas.