This is twice (I think) I've seen this guy. Both at night with lights off or blue lights only or early AM when I'm leaving for work.
I'd say it's about 1/4-1/2" long, though it seemed to strech at least 2-3x that, as it was gliding over rockwork. And, with dim lights was hard to tell coloration with shades/shadows, and the like, but it seemed a cream color or light grey.
It was some kind of snail, or slug.
On it's back, there was this oval-like disc type thing, from what I could see. And, hanging down from it were very small filliments or cords, or some kind of thing. Three or four of them. It almost looked like some kind of shell, but it wasn't a snail type shell as it seemed far too small for the elongated body to suck back into. This 'disc' was dark, a very dark grey, black, or dark brown color.
I've only seen it on the rock.
This morning when I saw it was just slowly going around. It seemed more like a slug to me, as it would extend it's head/upper body and elongate it with a curious sort of elasticity that in marine life I'd only seen in the lettuce nudibranch I put in my tank, but this certainly was not my nudibranch.
It didn't match any regular type of marine snails I'd seen before; it's no nudibranch, and it's not matching up with any pictures of a regular CuC that I've ever seen.
I know it's probably not the best description, and pictures are better -- but, as I've yet only a camera phone and was on my way out the door to work, didn't really have time. I think last time I saw this thing was probably a few weeks ago.
I'm not too worried about it, just more curious. Any idea what it is?