I'm not sure what these are, but there are two main areas that are literally teaming with small "critters"

They move around in constant motion under the shelf in the picture and in a series of holes in another rock that I cant get a pic of. They will scatter under direct light, but otherwise they stay in those two areas.

Got any ideas???

I have tons of snails, hermit crabs, emerald crabs but everthing else is alone. The only other thing I can think of that might be reproducing in here is the Berghia's I just added, but these little guys ar definately free swimmers.