This video says about itself:
This Bizarre Sea Creature is Snake-like and Has Tentacles | National Geographic
25 July 2017
A diver filmed this bizarre sea creature at the bottom of the Red Sea off Egypt. It’s likely its species is Synapta maculata—one of the world’s largest sea cucumbers. They can grow to be seven to ten feet long, and they filter feed by capturing floating organic matter with their feather-like tentacles.