Just found them the other day as well. Concerned me as there were a bunch of them on my frogspawn up at the base of the live tissue and all over my rocks and zoas at night. Pulled one for microscope check and they are ostracods, just like your pics.
From everything I have read so far they are harmless and should be treated like another beneficial pod. That being said, there are plenty of reports of amphipods eating zoas so who knows. There are hundreds of species and maybe some may not be 100% good.