is that a majano or tube anemone?
at any rate...pull the zoas and give them a quick fresh water dip..(match the temp and pH of your tank) if there is any nudis on them they will fall off...the critters you describe sound more like amphipods, and they are cleaning up the dead/dying zoas...nudis are pretty slow compared to amphipods...
if you have nudis, there will be spiral like white egg sacs attached to the bases of the zoas...you will also want to look for zoa spiders (by far the nastiest looking little bug)