I have a Starry and I love him. Never can be sure, but I haven't seen him bother or nip anything other than some Maiden's hair that I have in the tank. I do not have any Zoa's though...
I have seem him rest on my Derasa from time to time (not recently), but I have not seen him bother the clam.
He has a LOT of personality and is always looking for me when I feed.
Vivid Aquariums had a number of them a few months ago, not sure if they sill have any.