I had about 100 of them in my sump one day i started picking them off the glass each time i did a water change.. i have no clue what they do but havent harmed anything to the best of my knowledge.. i still have a few scattered on the underside of caves n such and in the sump.. i dont think they like the light too much. a couple of times i saw them attached to my zoas i just picked them off
they look cool, It is nice to see little living things in the tank. Do they need good water quality. I also noticed I have these grey and black tenticals poping out of a live rock and it has a baby feather duster comming out as well.
Its in most tanks....they are good, will take care of themselvs. Just leave them alone...yoyu will find them ostly in your sump, overflow, other places that rae in dark/shaded areas, and not bother by fish/inverts/corals/etc