OK, I give up....what is it? It lives on the roof of a cave in my live rock about an inch above the SB. I can't clearly see where it houses itself but it is definatly in the rock. It never moves from where it is. It never appears during the day even though it is dark in the cave. At night, when the lights have been off for an hour or so, I take my trusty flashlight and see what looks like a siphon tube, about 1/8th of an inch in diameter sticking into the SB. As soon as the light hits it, it retracts back and disappears into the rock again. I cannot be certain how long it is but I would guess 2". It is a light grey color, and seems to be very photo sensitive.
You can never have too many friends.