It doesn't move, but it's nocturnal... just what does it do that makes you say it's nocturnal? It looks like a shell stuck in the rock and maybe a sponge is growing in the gap between the rock and the shell. I'm not saying that's what it is, I'm saying that's what it looks like from the photo and the small amount of info. Have you poked at it with anything? Does it react to touch? Is the silvery crosshatched part look like a shell? Is it hard? Is the black part under the 'shell' soft? Do the hairy things at the bottom move? While I was writing pagojoe offered the idea of a clam and that makes sense to me. I see them 'stuck' inlive rock all the time when I snorkel in the Keys.