Jackie,
If the moss was growing on the rock, that is, attached to the rock then I think that if put into partial shade where it gets a couple of hours of direct sun, and is watered with rain, distilled, or r/o water,daily the moss should come back. If it was just draped over the rock then the probability is that it will not unless you remove it to full shade upon a substrate that drains good and stays moist. I use a brown paper bag, folded down as a substate and any plastic cleaned out tv dinner tray, such as for lasgna, to put the paper bag, cut to size into and put the moss on top of it. Water heavy on planting. Spray and/or water with distilled or r/o water daily, if dry air then 2-3 times daily depending on air temps.