But that is all floating stuff...not as substantial as squid or shrimp or something. (OH, CORRECTION - just noticed the krill so that is something larger :yes: )
Are they spot fed or left to scavenge? IMO, most of us are worried about water quality and are heavy on the clean up crew. That wouldn't leave much food for them, IMO. Not that spot feeding prevents predation, but if they aren't, I can see where they might very well be hungry and following instincts. Still, I would again expect you to notice a huge distension of the disk of the stars, possible damage to the disk.
What color are these stars? The green is a known predator....
Anyway, if you are suspicious, return them to the LFS, or they will work in a refugium.