well,,, it is rock that has been in water for awhile and the dead stuff falls off and new life starts to come to...... What I do is have a seperate tank with good water flow in it and let it in there for about 2 weeks (lights off) and then change half the water and start giving it about 6 hrs of light a day and you will then see the coraline starting to come in (purples,reds,...ect) I like to set up a new tank this way because the rock helps cycle the "NEW" tank .