Sorry I don't have a picture of the critter....but what I found looks close to the picture i have here. I can describe it fairly well.
It's about as big around as a pencil, 1 inch long. Where this one in the picture is red...the one in my tank is more clear or creamy colored. But it has the tentacles (if that's what they are) attached at the head. I see similarities, but the body is a little more long than fat like this one. It came in on a rock with some polyps, and I saw it there, but when it was on the rock, it looked black with a dark blue, velvet looking head. When it finally moved off the rock, it was all neutral colored, except for the attachments at the head of it, they were all white. I've only seen it off the rock this one time, it was on the move, and haven't seen it since. Any ideas?