I see it a lot and if it looks normal (not skinny or white or otherwise strange) but long I don't worry. Platys eat all the time in a planted tank picking at algae & plants etc., and are pigs at feeding time. I deworm mine when I get them ( praziquantel, metronidazole, levamisole during QT-not all at the same time) as they do often come with worms and parasites. If they are plump and act healthy with no clamped fins or flashing and no visible worms I don't treat them again. I have heard constipation can cause this-I include a lot of veggies like zucchini, dried seaweed, spirulina flakes, omega one mini veggie pellets/veggie rounds and frozen daphnia and avoid a lot of meaty offerings with them, but I still see very long normal poos often.