<a href=showthread.php?s=&postid=7102903#post7102903 target=_blank>Originally posted</a> by GMAX
Can you give us some perspective on how large this is. Without anyreference I would say it appears too large to be a flatworm. Do you have lots of them? Flatworms tend to cluster in places and there would be enough that if you look closely you would seen a cluster on the rocks.
I think you have a copapod.