I've noticed my population of those and my over pop of feather dusters have declined. Maybe you just need a Harlequin tusk or some other hungry fish since seems to be happening to me. (And I actually dont mind massive repopulating cuc wru micro brittles)
Might have a game plan in play for post starfish reduction. I've always read unless this is like 1 shrimp in a 300+, they will do such a good job decimating them theyll end up starving.