Emily you always seem to have something interesting going on! You've definitely got a blue thumb.Next time you are in Vancouver I'll get you to "bless" my tank
What you have looks like a
Asterina sp.
maybe
Asterina burtoni Generally they are considered harmless. Many of the species range from 20mm to 50mm so they will stay small. You have a "bloom" of them because either one divided by it's oral disc, or more likely the parent star was brooding young. Little is known about their reproduction so They may be able to reproduce asexually. They are noted as general detritus feeders and shouldn't be a threat to anything large. You may find that wrasses or larger fish pick off the odd one.
Anyways, if you are looking for new homes for them let me know. I've always got a place for little critters. If you can get something closer up I will check more sources.
So what will you post next week?