Where the fish completely black when you got them or did they have the white on them at the shop?
How long has the tank been set up for?
What is the GH (general hardness) and pH of your water supply. This information can usually be obtained from your water supply company's website or by telephoning them. If they can't help you, take a glass full of tap water to the local pet shop and get them to test it for you. Write the results down (in numbers) when they do the tests. And ask them what the results are in (eg: ppm, dGH, or something else).
Mollies need water with a GH above 250ppm and a pH above 7.0. If the water is not hard enough or has a pH below 7.0, that will stress the fish and they can produce excess mucous that looks like a white film growing over the fish.
My guess is it is just colouration and nothing to worry about but check the water quality for ammonia, nitrite, nitrate, pH and GH and post the results here.
