Sorry I don´t have a proper answer. I would be guessing it´s due stress, those doesn´t like real slugs.
Something that can be compared to this has happen to me. They didn´t look like this ones thou but one of my females poop looked to contain a chameleon curled tail, several dooses like that. I am still to this day unsure if it was just peculiar shaped poop or unfinnished developed jackson noenates.