When you mix aluminum and mercury, you create what science types call an amalgam reaction. You have to get past the oxide layer to make it go, and in this video, that happens with the help of acid. But once it does and the aluminum is exposed to oxygen, it starts producing flakey, creepy new layers of aluminum oxide, which is what you're seeing rising up like a pale fist from the colorless depths of a Stygian floor tile.
You can't do this very easily on your own, since you need to get through that oxide layer to start, and that requires the use of acid, so you're doubling down on terrible poisons to try this in your own home. Please don't.