If unable to do the above could you describe the effected area?
Is it just near the tail fin or is it in other places? If so where? Gills, eyes, whole body etc.
Does it resemble white individual dots (like sprinkled in salt)?
Does it resemble a stringy cotton ball?
If mollies are the only fish in the tank you could increase the salinity a touch before getting a definitive diagnosis. 1/2 teaspoon of aquarium salt per gallon of water. Might slow progression if it's parasitic in nature but won't have a negative/positive effect if it isn't.
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