What are these? (Intertidal, Malaysia)

These creatures were found attached to the underside of a piece of sponge in an intertidal area in Malaysia. As far as we could tell they were anchored, don't change their position but they appeared to be twitching or "ticking".

I've ruled out the pods (isopods, copepods and amphipods). At first I thought ostracods but they don't have that forked tail. I thought maybe triops but I've read those are freshwater. My next best guess would be clam shrimps or something in that Branchiopod clade but I'm very unfamiliar with their morphology. Or maybe the juvenile stage of something?

Anyone know what these are?