I haven't seen the creature on that picture, but once I saw a small "jellyfish" in one of my fry tanks. It was round with a diameter of about 1/4 inch, with the appearance and movements of a real jellyfish complete with short tentacles.
Hey, I just saw the other pictures on Oliver's site and it has the jellyfish listed - Craspedacusta sowerbyi - photos, info.
The maximum size is just under 1 inch so this jellyfish can't look too good in a normal size planted tank. But I think it may be an interesting addition to a nano tank
That is a freshwater Bryozoan, and is not in the phylum Cnidaria that contains the jellyfish. So, it isn't a real polyp. The polyps of Craspedacusta sowerbyi are said to be very small and inconspicuous. I had a friend once who did research on Craspedacusta, and who said that the creature's name when screamed made a wonderful war cry---Craspedacusta SOWERbyi!!!