The salesman is not even close. The critter you have pictured is a colonial tunicate. I'd be leery of taking their livestock advice on future purchases. There was a thread a while ago on this exact same creature, except that time it was called a "lollipop coral" if I recall correctly. A search of the board should turn up some more info for you.
If you're really interested in tunicates (sea squirts) a new book has just been released on their keeping and identification. You can order here: http://www.dynamicecomorphology.com/
On the left, click on DE PUBLISHING and then click on the "Ascidians(Sea Squirts)" book. It gives you the table of contenst, pics of pages, and I believe the entire introduction. If you look at the cover, the bottom right picture is of the tunicate in your tank.
I should be getting the book on sponges any day now...
Thanks all! So it is a kind of Ascidians / Sea Squirts /tunicates.
"These benthic animals live a quiet life pumping water through their branchial or feeding sac. To capture plankton, bacteria, particulate organic matter and collodial particles the ascidians construct a mucus net or mesh that has pores which are microscopically small." by The Captive Oceans Volume 2 The Ascidians
Also from other sources, this tunicates is hardy; with low light requirement and to use DT to feed.
As far as I know, tunicates are not photosynthetic so that would be a no light requirement. I'd stock up on the phytoplankton, pick up some liquid life bioplankton or some tahetian blend in additon to the DT's since these guys are strictly filter feeders. The more the merrier. HTH