What's wrong with my fish?

I have a Neon Butterfly Molly fish that I recently got at PetCo. She is alone in her 5 gallon tank and 2 days after I got her, she had fry. My question isn't about her having fry, but about this white string she leaves all over the tank. It's not fish poop - I can tell the difference. It's a solid white color and she will go around and try and eat it sometimes too - YUCK! She is also hanging around the bottom of the tank a lot. Is she okay? Any advice?