White stringy poop indicates stress, mucous discharge from intestine, bacterial and parasite infection. Take your pick there. You have to backtrack your tank's and fish's history to determine the exact cause of this issue. Water parameters, introduction of new fish (quarantined or not), food, temperature, color of gills, other symptoms, etc.
I suspect you are even more informed than I am on the subject.
I've never seen a proglottid or suffered any of the symptoms of an infestation by common human hitchhikers. But when I fasted for three weeks, white poo. Looked like intestinal or stomach mucous sloughing off. Pretty sure it was irritated from the excess acid being dumped in to stimulate my appetite. Wouldn't fish innards experience similar tribulations in the face of prolonged fasting?
For some reason, that makes me want to see some white human feces.