My fish were sick for a long time before I figured out what was wrong, and didn't show outward symptoms...the first sign was that they weren't swimming around as usual, and preferred to hide out all the time. It took a while for them to heal. One fish never really did...ended up pretty deformed from malnourishment.
Now, I didn't say that that was what your fish had. Just suggested it's something for you to consider. It's not typical for mollies to hide from the light.
It is possible that your new fish came with parasites (some fish species often have internal parasites when you buy them.) The tough thing with internal parasites is that it's really hard to diagnose, but treating for them isn't too difficult...some aquarists prophylactically (and periodically) treat their fish for internals with medicated food, especially when new fish are added to the tank.