Huh, I just learned this through hypothesis and experiment, nobody ever taught me anything about xenia. I've got some directly in the line of a powerhead and it gets tossed around like it's in a hurricane but it's super fluffy and if I turn the flow off, you can see that it pulses like crazy. I assume it also does this when the flow is on but the thrashing from the flow obscures it. I've got some around the back of the tank that migrated itself there, it's very thin and doesn't pulse except very very weakly every couple of seconds, basically just twitching. This one grows slower than the high flow one and looks sickly, so I decided to take a frag of it and introduce it to medium flow (gentle sway back and forth, like a hammer) and it looks and pulses like average xenia.
It took me like 8 months to do all this so I think it's pretty solid, and it all occurred in the same place so there was no difference in water params from specimen to specimen. I'm not sure if the amount of light available matters, that's my next experiment