I've seen these in the wild, creepy as ****. They also stick to your wetsuit and are a nightmare to remove, have to use a knife to scrape them off. Come in all sorts of colors.
They require a lot of food. I could only keep one for a few months. If it's underfeed, it will start to lose it's limbs. The feather-like strands will fall off first. They also like to move. Cover your circulation pumps. Good luck...