Michael- They look like they would fit somewhere in between the 3rd and 4th stage on that link. They really do look a lot like an adult except they are clear (except for their silver eyes), have exagerrated limbs (i.e., they have what looks like raptorial appendages that fold just like an adults, but they are bigger in comparison to their body size), and their eyes are spread farther apart than you would expect of an adult.
I wouldn't absolutely rule out that it could be something else, but the rock has been in my tanks upward of 5 months and I've never witnessed anything like these before and the only other tank inhabitants are snails, hermits, and a damsel. Someone said they might just be pods, but they are about the size of an adult amphipod and I think it would be very strange for a swarm of 100+ adult pods to just show up all at once like they did.