Can I ask where you're located? I live on the gulf coast, and I'm always checking out the local marine life, and that looks exactly like an anemone that I've seen. I don't know what it's called, but if something bothers it, it turns into a little brown ball.
I kept one for a while with my first try at a salt water aquarium, and it was very hardy. It was also very aggressive. It caused chemical burns on anything that came close to it, and successfully killed both fish and shrimp.
Although I later returned it to where I found it, I was pretty impressed by it, as well. I usually keep local cleaner shrimp in my tanks because I like the way they look and their cost (free) and because they're from local waters...but I couldn't keep them with this anemone. They were eaten too fast. So I started buying live gulf shrimp to feed them. My little one (started out about a 3/4 inch button) could eat an entire 6" shrimp in under 30 min. It was amazing to watch.
I released it about 6 months later when it was I caught it trying to devour a 4" Sargent Major, (another local fish).
Anyways, if anyone's interested, I've got a few pics somewhere, and I can try to get a pic of them in the wild.
Huggs,
Dyra