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It could be nutritional deficiency (vitamin C I think), or parasites as well. Mollies also do well with salt in the tank. Do you add salt, if there aren't any other fish that can NOT tolerate salt? Also some other illnesses.
One of my Swordtails has a bent spine and I gave the fish deworming flakes and the fish straightened out pretty much was still a little bit off but not bent like a lazy S as it had been. She lived on for a year or so maybe which seemed to be more than it could have been.