Well, because of the wild caught history of the animal I would look at this as suspicious but it doesn't really have the appearance of slugs or unfertilized ova. It may well be a form of diarrhea based on the sole fish based diet. I would consider scenting some small pink mice with the tilapia to get her to take some of that. If she does, then continue to observe the waste to see if the consistency changes. If it remains soft and watery then I would consider taking a sample to my vet for testing.