Turtle I haven't seen this in turtles but I do wonder if you've thought about parasite testing. I've seen gelatin type waste in other animals who were infected with parasites.
Next I'm wondering is there any chance this turtle has eaten something it shouldn't have? Does it spend any time outdoors in the pen where birds could possibly have dropped something in that the turtle shouldn't have eaten? I have that problem.. the people next door feed scraps to the birds which I find dropped into my yard and 3 times I've found my turtles trying to eat varius types of bread. I've even found bits of sweet rolls with sugary frosting on them, yikes!
I've seen what you describe in animals who had an enteritis condition which can be caused by various infections or injuries that of course require a vet to diagnose and medication to fix.
The injury that caused this was caused by rock eating. The animal ate rocks which caused ulcers in the stomach, however that animal's waste was tinged with blood.
Allergies to certain foods can also cause this, even though allergic reactions are more common in mammals anything is possible.. especially for a captive reptile.
If it doesn't clear up on it's own I'd see a vet but whatever you do please let us know what happens.
Good luck with this!
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PHRatz