Unfortunately, there are A LOT of causes for this and fish are difficult to diagnose. I assume you very recently acquired the fish, how long have you had them in that tank? Also, where did you get them (Local fish store, big box pet store, private sale)? How often do you feed and do you give a mixture of those foods at the same time or do you alternate them?
Here are some of the most common causes of white stringy poop and why I asked those questions. The #1 cause is stress and sudden changes. A new environment with a sudden change in diet both can cause it. It May just need more time to adjust. A diet that isn't balanced well takes more time for the fish to acclimate to the new food. I have noticed that a diet too high in protein and low in fiber causes the stringy poop to hang on a long time causing a long string of white poop.
Another cause can be an imbalance in the fishes hormones and enzymes, causing the digestive system to be either over active or under active. An imbalance can cause all kinds of symptoms and strange poops. I find this to be the least likely cause in your case.
It's also very possible that it could be a parasitic infection. Many times, the parasite either consumes a lot of the fiber and or protein in the gut and causes the fish to poop essentially an empty white caste instead of the normal waste. Other times they feed on the fishes blood supply to the gut and it is not properly digesting. Parasitic infections also often cause a distended abdomen which I can see in your photo, but it could also be distended for several other reasons or could just be fat! It sounds like you got the Molly together with others. Do you know how long they were tank mates before you got them. If it was a considerable amount of time and the other fish seem fine, a parasitic is less likely but still possible.
To be safe, I would quarantine the fish for several weeks to rule out a parasitic infection that can pass to tank mates (if it hasn't already). You may find the fishes stool improves when it adjusts to the new diet and feeding schedule. Then you can return it to your main tank. Avoid medicating your main tank. If you want to medicate the fish just to be sure, do it in the QT tank and use a medication that isn't too extreme. If the whole issue turns out to just be stress, the move to QT and medication will only make it worse.
I hope this was somewhat helpful. If it was me, I would just QT and monitor for improvement. If no improvement in 2 to 3 weeks (which should be more than enough time) I would consider medication such as Fenbendazol. As a wide scope dewormers.
Keep us updated on,your progress and if the fish makes it out OK! Good luck!
