hopefully it is one. at first to looked manmade it was so uniform and thought maybe knob star. but when I went to touch it it retracted. if it is, spot dose with phyto? I have an order of Dinkin's coming this week.I'm betting tunicate. I have something very similar from my TBS rocks. I can't see from the angle, but mine has a larger siphon hole and a smaller in the back. They do pretty well as long as they don't get covered by anything. Valonia can stick to them and cover them up, so best to keep Valonia in check. Overall neat little addition to a tank.