They will pulse when they are grabing food form the water. What are you using to feed them?
It is believed that Xenia is incapable of feeding, and that it pulses to center the light at the center of the colony. There are many different theories to its pulsing, but I can assure you they do not eat. They get all there food from the light. It should pulse if it's happy. They say they either grow like weeds, or they just die. My frag
was pulsing 30 mins after adding it, and it wasn't even attached.