Something pulling shells/frags into rockwork and making strings?

[STiTCH87]

For awhile now i've been losing snails and hermits and their shells have been collecting in one big mass in one area near an opening in my rock work. The shells are broken up and also held together in a bundle by these little white thread looking things almost like webbing (think the strings of a clam kinda deal). Not all the shells get broken but most do. Well for the past 3-4 months now, that sand bed frags near that area keep getting pulled up into that crevice also. I have NO idea what's doing it. I don't have any hermits large enough to carry frags anywhere. I don't own turbo snails. And I never hear clicking and my sand bed never has hills so I know it isn't a pistol shrimp.

The only thing I can think of is that last year I had seen a eunice worm in one of those rocks (haven't seen it since) and also I have a tiger sea cucumber. But I have never seen the cucumber move frags anywhere, and he goes all over all of my tank so to think he'd only move the same area of corals and nowhere else in the tank is unlikely. Whatever it is, I feel like it lives either in the rock work, or specifically stays in that same area it's whole life. I don't have the money for a surveillance camera that can record 12 hours while i'm away, so I need suggestions.

I think the white strings and bundle of shells may be our best starting point toward pin pointing what this could be.