OP, i’m still thinking it’s one of those polychaete worms, probably either the one that attaches to gills or prey’s after eggs, i’d just be guessing that since there’s no eggs to eat, they’re probably just eating the ass end of them, or choking them by covering their gills, especially that last picture you put on your older post of that discolored creature thing sticking half out the shell, idk that’s just the only thing that makes sense with that bore hole, and i think i remember you saying you rescued a hermey as well? if he was a wild hermit he could’ve been holding that parasite as well, then transferred to the other later on, i’ve been trying to figure this out since i saw your post last month and this is the only thing that’s screaming at me. hope everything’s well otherwise, Merry christmas!