This is going to be one of the most bizarre food items you will ever see. The squid isn’t alive. It’s been decapitated. The legs are just reacting to soy sauce. The dish is call odori-don:
A live squid with its head removed is served on top of a bowl of sushi rice, accompanied by sashimi prepared from the head (usually sliced ika (squid) and ika-kimo (squid liver)) as well as other seafood. Seasoned soy sauce is first poured on top of the squid to make it “dance”.