Not all Xenias are the pulsing kind. I have heard that they like systems with some Phosphate and Nitrate. In fact some keep them in refugium and harvest them just like macro algae.
The ones in the reef I maintain were huge until i switch to an ATS and the Phosphate dropped off. I started feeding heavily and they slowly came back. So I think there is some truth to the 'dirty water' theory. That, and I was told that if you run an ATS and get to true zero N and P that the Xenia will likely die.
Other than that, I would think that as long as they have enough light and are not in heavy flow, they should be pulsing.
Oh yeah, I've also heard that they have a finite life span of about 2 years. If you frag them by just slicing off at the base, they grow right back, so frag them whenever someone else wants some. I've heard if they die they will emit toxins so watch carefully.
My tanks: 120 Reef w/L2 Algae Scrubber, 60 Reef Pond w/floating Algae Scrubber, 40 Breeder Reef w/L2 UAS Tester Algae Scrubber
I maintain: 144 SW Reef w/L2 Algae Scrubber | 200 SW Reef w/L4 Algae Scrubber
Special knowledge: Algae Scrubbers, Acrylic Fabrication