I dumped my rock with a friends rocks that were mixed with local rock that had gotten from the ocean... I knew there was something in there but couldnt find it and never saw it. A year and a half after putting my rock in, I happened to pick up a rock, and out rolled this little log looking thing that was like the size of a finger, that immediatly jumped up and went from brown log, to mean fighting crab with crazy killer claws outstretched, scurrying under a rock. That crab ate so many firefish, gobies and other small fish.... It wasnt a surprise to find/see him finally but did it make my blood boil!
Good luck getting them out. Suggest to just put a crab trap in there and keep catching them! Really is great looking rock!