Yep, those are regular, basic bristleworms that are either hiding under the nice cover, or have come to eat the dead flesh.
Chalices just do that sometimes if you upset them sufficiently. They can go from looking okay to a heap of goop with shocking speed, particularly if they get damaged in shipping. Which they often do- their sharp skeletons cut them up from the inside.
(Edit: don't know if yours was recently shipped. It just came to mind as something I've seen them disintegrate from. You get a chalice that looks a bit damaged, think "oh, it'll probably heal up fine, LPS are good at that", and then, no, there it goes.)