Thank Chaloupa, I have identified him as an Asterina star. He doesn't seem to be doing any damage to the Palys but I should have removed him when I found him. Ah well, if my Palys start disappearing I'll know why at least. I've read that they are hit or miss as far as being reef safe, so I'll keep a close watch on things.
I should note the frag is closed up in that pic because the hermit had just spent some time crawling on them, cleaning between the polyps. They opened up really fast after that pics was taken.
EDIT: He's not on the Paly's today, I inspected
So, who knows?