Mysis

[ferret]

Those are mysid shrimps and are very common in the display tank when placed with live rocks ( I think they came along with the LR ). However, their life cycle is short, may be less than a couple of weeks. If not being eaten by the predators like wrasse, they can regenerate quite quickly and steady.
A perfect live food for the seahorses and pipefish other than amphipods

[Starry]

regenerate = breed?