Hard to say what it is other than this is not a normal stool.
Is it alive, does it move on its own?
Could simply be stringy kind of colorless poop that is often indicative of intestinal distress, including that from parasites.
I'd give it Epsom salt - magnesium sulfate - with feed. That allegedly cleanses all intestinal parasites. If not eating - administer through baths, albeit it'd be less effective but still a viable medicating approach.
Same with other general antiparasitic treatments - praziquantel, metronoidazole and fenbendazole are the ones I chose. Again, best with feed, can be combined all three. If not - through tank water long term or baths short term.