You may well have found hundreds of larval stomatopods. When a female broods her eggs in a cavity, the eggs hatch after three weeks. The larvae then remain with the female in the cavity for another week. At that time they molt again, become photopositive and swim up into the water column. When you dumped out the rock, the larvae may have been washed out of the cavity. They do not survive in an aquarium (little or no food, filtration) and will die in a couple of days. They must remain in the plankton for a month, so there is no change that you will be over run with juvenile stomatopods. It just does not happen.
Roy