My 2 cents, applicable only to the white pulsing xenia, pom-pom:
I keep it in all lighted tanks as an alkalinity and toxins indicator (although cynarina is more sensitive to everything, my impression, but too big).
When alkalinity drops below 7 dKH (happened few times), xenia movements became lethargic. Srinked, when toxins were in the water.
Still pulses:
- at pH 7.8
- NO3 80 ppm and PO4 1ppm
- Ca 360-460 ppm
- when iodine was never added, water changes only,
- what else?
I'm not keeping such parameters on purpose
Water flow:
the best growth, thick short stalks, many new stalks rooting - with flow around it, max tried 25x tank volumes per hr. Min - flow trough, the branches are not moving. If in the stream, even below 150 gph, - becomes leggy.
Color darkens in the dirty tanks (at least it seems so).
Light:
slowest growth was under 18W 50-50 PC for 6 g tank, fastest - in 7" below the 55W 10,000 PC, high nutrients tanks. Have xenia under direct sunlight too, but less nitrates and phosphates - same rate of growth (by eyeballing

).
Tried xenia refugium:
moved it from the 18W (50-50 PC, 6g) tank to connected to it shallow container, same water is circulating through, only different flow pattern. The light as recommended by Melev's Reef for refugiums, fluorescent flood light with mirror side coating, 16W of lower Kelvinage (?) (2,900 K or 4300K - have to check) for 4g shallow container - stopped to grow, become darker, more contracted, but still pulsing.
I have suspicion, that at some point it was eaten by the fish - to the middle of the stalk - neighborly connected xenia continued to be fully open and pulsing. New polyps were grown pretty fast, just like new.
Once become wilted with very dark slimy coating, like mould, and dying. Siphoned away all dark slime coat (by baster), few times a day, all normalized in a few days.
Ah, and ruptured during the shipping xenia restored reasonably fast too, in 5g QT under 27W 6,500K CFL from hardware store, 20+x flow.
I would vote for keeping it as a general indicator, if some corals are feeling bad and xenia is good - things are not too bad in the tank.
Hope, that helps.
PS: Cynarina is better for a purpose, but 8" in diameter.