That is tube coral -- Cladacora Arbuscula -- I got several clump as hitchhikers on my LR. When it arrived, it was encrusted with red coralline algae -- all of that is now gone, and the clumps have been attacked by crabs, which accounts for about 1/2 of them dying. However, a few clumps are still doing well, and one even dropped (or spawned) a baby clump which has since been buried by Rain. I have no idea if it's still alive. I haven't found it for sale anywhere, and I'm not sure that it would be a popular tank species. It isn't particularly dramatic, though it is nice to watch when the single polyps are extended.
What is the coral in front of the zoas?
+1 on the built in light. My cheato is in the tank, stuffed into a back corner where the peppermint shrimp and goby seem especially fond of it. I can see how nice a lit back area would be, though.
I'd like to chime in on this. I recently had the same dilemma about what to put in my chambers. A good piece of advice I read was to find a tank you like and copy that person's method. So just this week, I put a filter/polishing pad in chamber one, then chemi-pure elit and purigen, then I have the heater and pump in the last. I have plans to add chaeto but don't have a light for it yet. It's nice that you have a built in light! Definitely take advantage of that. Good luck!