In my experience they usually only extend when there is something nearby it wants to eat. I had an acro beside one for months (about 2-3") and then out of nowhere one day it looked like a big spider web was covering it that was coming from the hydnophora. That acro was toast.

Good to know. I moved mine a bit farther away from the gorgonians.

FWIW, I had blue mushrooms once that killed my hydno, so some things can get closer. Just not other SPS :)

I think mushrooms can kill anything really. Mushrooms, Green Star Polyps and Xenia seem to be indestructible and grow right over other corals.

Only if mine touched something did you ever see sweepers(1 1/2"+). And man could they ever cause damage.

hydno
 

my 8 inch square fluorescent hydno never hurt anything as nearest thing is 1 inch away from it. i find that if other corals ie: monti, poci, most acros. birdsnest etc that dont send out sweepers can live almost within touching distance of hydnos.