What is the temp in the tank, and do you use proper lighting? Quite often they'll turn a dull grey color if not given adequate light, or none.
Could very well be a slime coat issue like another said, without it, or with damage to it, it opens them up to parasites that'd otherwise be no problem. Mollies prefer brackish water settings, but can thrive in a properly cared for tank. They do prefer some salt though, and it's so much healthier for them!
If the temp is too low the molly is open to a lot of nasties, it lowers their immune systems if the temp is below 76 and will eventually kill them. Best to be kept at 78-80 degrees. They also like to be in groups and need a tank no smaller than 20 gallons for the type that stick around two-two and a half inchesthose are the smallest ones), the bigger types like sailfins and lyretails need 25-30+ gallons to be happy.
What were your water stats? If you had -any- ammonia it can hurt them. Same with nitrites. Low nitrates can sometimes be tolerated(lower than 15ppm), other times they can't.
Is the tank cycled and properly filtered? If you don't have a proper filter, there's not enough oxygen and too much filth. If the tank isn't cycled yet, it explains everything, even if you use "safety" chemicals. I had a silver molly that did this when I didn't know about the cycling process, put the poor dear in an uncycled tank, she got a bit grey and would hang on the bottom and kinda wobble a bit.
Hanging on the bottoms is a sign of several things: Water too cold or too hot, not enough oxygen in the tank, toxins in the tank(like not being cycled yet), mollies sometimes do this when they're constipated or going into labor(a pea will help if it's constipation, but this sounds like either a really nasty case, or something else altogether, they don't normally turn colors), a really bad swimbladder infection(caused by a dirty tank most often, but some are just delicate and get them, a cooked skinless pea will also help with that), and a sign of just all around illness.
Check your stats, post everything up here. We need your ammonia, nitrite, nitrate, PH levels, hardness if you can get that for us via a testing kit or your water company(if the water is too soft that can also be a problem, but it'd have to be pretty soft or changing still to cause big issues), as well as cleaning done and how often, light used, chemicals used for the water, how long you had the fish, what size tank, and how many others you have(mollies need groups of 4-5 minimum, they also have a high bioload and need good filters and a good bit of room), plants and anything you added in the last month or so, etc. =) We can help better if we know what's what on your end. Need some more info.
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