This is an Umbrella Octopus

Opisthoteuthidae are a group of octopuses characterized by a web of skin in between their tentacles. They have a U or W shaped shell that holds the mantle and connects to their tentacles at the bottom. This structure makes the umbrella octopus resemble an umbrella when they spread their tentacles/web out. The structure of the umbrella octopus has the oral surface below the mantle of the octopuses and the web with their tentacles surround the bottom of the mantle.[1] Their outer skin has a very delicate consistency that results in white spots appearing on their skin when damaged.[1] Opisthoteuthidae fall into the cirrate octopod group, meaning they have fins.[2]