Really weird looking hitchiker

[Palmer]

I have had some live rock in my tank since the begining of January and it has been cycling without a light until the last couple of weeks. Today when looking at my tank I noticed this large white thing move in my peripheral vision from the surface of the rock back in. I looked in the hole it went into and could not find anything. So..... I thought it might be a worm and started looking at all kinds of worms common to the hobby and since I didn't get a great glimpse of it I came up empty. Then after the lights had been off for a bit I went and turned the lights on and this thing was sticking out of the rock that had a white head about a centimeter across with what looked like two antenna (that were independantly moving over the rock) and the body that was going into the rock was brownish. As soon as I turned the light it quickly went back into the rock.

What is this thing?! I have heard of snails without shells but are they this big and do they quickly move in and out of rock? Could it be a worm?

I will try and get a pic but I do not have a great camera and this thing is really skittish.

Palmer