Asterina starfish. There's a lot of different opinions out there regarding these things but I for one don't rust them at all. If one pops up I'll remove it ASAP.
Asterina starfish. In small population can be accepted. Have a few in my 90 and my 20 with no ill effects. That said, they can potentially get to plague proportions. I have had no issues with my white colored ones, but have heard the brown/tan ones are a bit more prone to snack on some softies. (I may be wrong, but that's my .02)