Denise hit the nail on the head. Xenia directly absorbs dissolved organics from the water. I've seen huge beds of Xenia near sewage outfalls in Indonesia. The other factor that effects their pulsing rate is pH. The higher the pH, the faster they pulse. My Xenia have been going crazy since I started dripping Kalk. I think the reason a lot of reefers have problems getting Xenia to grow is that they are over skimming or using a lot of carbon. All they need to thrive is dissolved organics and lots of light.
I don't know that much about the Pom Poms. They tend to be a lot more finicky than the common elongata.
Gary
Gary
125 SPS, 75 gal. LPS/softie reef, 9 gal. Nano