Dead liverock, now thats a oxymoron (sp?)......I seriously doubt its really dead. Its not necessarily the critters you see "on" the rock as much as the microscopic critters that live within the rock that makes it live rock. Could be the fish nipping or grazing on the critters that used to inabit the external area of the rock....feather dusters can die if they have not gotten proper foods, and it al depends on how and what you were feeding the fish, as the dusters may not have gotten sufficient nutrition, but they can go for a long long time with little to no foods so it seems. Other than no tiny feather dusters, have you been keeping parameters where they should have been all this time? Just arbitrarily adding cacium when you see a problem is not a cure......If you have goodlights (not old bulbs etc) been keeping the parameters of the water where it should be, in regards to alk, ph, Ammonia, nitrate, nitrite, and calcium, doing water changes, suffieint lighting schedule and keeping temp within limits and have not added any of those miracle concoctions sold in the lfs, your rock should still be doing good. So tell us what has changed......something has had to change somewhere?