I have a 40 gallon saltwater tank that contains not only corals, feather worms, live sand, a few fish and living rock, but also something I have no clue as to what it is. It appears to be contained in one of the pieces of living rock and doesn't appear to be coming out of the rock. You have long tentacles that come out of the hole and appear to sift through the sand and over the rocks and then later a large spurt of dirty sand comes back out the hole later on. Does anybody have any clue as to what this thing might be? I've had the tank up and running since last August and everything appears to be doing well, but would like an opinion as to whether this thing is friend or foe for the rest of the inhabitants in the tank. Any ideas as to what it might be would be greatly appreciated. All my levels are 0ppm Ammonia, 0ppm Nitrate, 0ppm Nitrite with a Ph of 8.2.