I have xenia and zoos in my nano seahorse tank. I feed the horses mysid shrimp every day, and have personally watched the xenia and zoos eat the shrimp that land on the polyps. I cut the pump off so that the horses can catch the shrimp, and they just land on the polyps, but they close up, and engulf the shrimp.
My xenia are growing like crazy, and I am going to have to frag it back to smaller size soon. I add no other stuff to the water, at all. I put 2 small frags of xenia in the tank about 6 months ago, and now I have 9 large and a half dozen tiny frags of the stuff.
I have two groups of the stuff, one very high up and one in the middle of the tank. The one higher up pulses much stronger than the lower level colony. Water flow is the same on both.
I am guessing that water flow, and light are much more important to these things, however, a little food every now and then can help. I would not add a lot of filter feeder stuff to the water because it can foul it up, in a hurry. I would just go with frozen mysid shimp, as everyone loves these and they are much more nutritious when compared to brine.