This video is called Atlantic longarm octopus, Macrotritopus defilippi, imitating a flounder.
From National Geographic:
Atlantic Octopus Mimics Flounders—A First
Published March 4, 2010
A Caribbean octopus has been spotted disguising itself as a flounder, most likely in an attempt to avoid predators, researchers have announced.
Over the past decade, several Atlantic longarm octopuses have been captured on video imitating the sand-dwelling peacock flounder, mimicking not only the shape of the flatfish but also its color and swimming style.
See also here.
Flatfish evolution: here.