I've been watching this thing for several weeks, now. FWIW, last addition to the tank was nearly a year ago. It's quite small (my pic actually shows details not visible with the naked eye); it perches on a rock low in the tank in the light, arching itself so it kinda resembles a limpet, but it usually stays in one spot for hours or sometimes even days. When it does move, it flattens out leaving a small bump in the middle, looking like a small mushroom with it's mouth puckered. It does seem to glide, like either a snail or sea star, but I'm not sure what's on the bottom. Since i first spotted it, it has never left that one rock, and has three favorite perches. Any clues? BTW, if you right-click on the photo and select "view image", you'll get a larger pic. Thank you.