Hard to say without a pic...could be brittle stars, worms...just about anything. Can you get a pic of it or give us a detailed description of what it looks like?...color, size, etc.?
I see tentacle looking things on mine as well; its kind of like a small flower with each tentacle maybe 3-5mm in length and the whole thing around 5-7mm in diameter. I can't tell if its moving or the movement of the water is causing it to dance.