Larval form?
My initial thought was that it was a larval form as well. I didn't see any antennae or obvious appendages, weird. I have not seen them the last couple of days but there were easily 100 or so of them swimming around.
The tank is 55g saltwater with liverock. The rock is about 12 years old (has been through moves and downsizing in the past, but has been maintained live). The current setup is only a year old. As far as critters go, I have 2 "skunK" cleaner shrimp, 1 pistol shrimp w/ goby, assorted snails, six-lined wrasse, 2 ocellaris clowns, and a yellow tang. I also have several soft corals.
I would have thought of the cleaner shrimp, but I would have expected the larvae to appear like brine shrimp nauplii or at least to have clearly defined limbs. I would add that my cleaner shrimp did have eggs at least a week prior to my seeing these little critters.
Thanks for the feedback.