Shouldn't be a an issue, as pretty much all reef rock is coral, either recent or older and overgrown. Just keep the skimmer and filter going and you'll be fine. I've even kept a couple large pieces from earlier tank disasters and used them for reef rock - foundations for the next generation of coral. Some of the frags I've picked up over the years simply didn't take and died, too. For those I couldn't tug loose I simply let them get overgrown first with coralline algae and then coral. Now they blend in as part of the reef.