It was about the size of a quarter, round, seemed like the top was soft, not a shell like a limpet, it definately didn't like light, I tried to shine the flashlight on it to get a better pic and it retreated under the rockwork fairly quickly. It looked like a brown mobile hairy shroom to me. What is it?
Yeah, thats a huge bristle worm next to it.

Yeah, thats a huge bristle worm next to it.