Im pretty sure it depends on what type of small starfish you have, I had small starfish in my tank for about a year (astrina's I think) 1000's of them. I also have a bunch of zoa's and I have never seen them harm any of my zoa's, but they ate my coralline like there was no tomorrow (leaving little white spots from where they were munching)
No ill effects to my zoa's in over a year. (I think it really depends on the color of the starfish, mine were a whitish grey color)
I finally got tired of them eating my coralline so I got a harlequin and it took care of them, in no time.
after my starfish were all gone (eatin up)
I had some zoa's close and slowly melt away for no reason, even after fragging and dipping. I had about 7 different frags of these pink zoas throughout the tank right next to other frags of different zoas and the pinks were the only ones affected.
Sometimes Sh*t happens