There are a lot of things that could be wrong. It's hard to tell without more information and pictures, but here are some basic ideas.
Mollies are prone to illness if they are kept in normal freshwater. Mollies need to be kept in brackish water, and I find that if they are left in fresh water, they start acting lethargic and wiggling a lot when they swim. Like they're too tired to move. If this fish had been kept in fresh water without any salt before you got him, he might be adjusting, and be in minor shock, from the change.
When you got him, did you quarantine him before placing him in the larger tank? He may have come to you ill, and this is the reason you should ALWAYS keep new fish separate for a few days, to watch for, identify and treat diseases.
Did you acclimate him to the new water? This should have been done as you where watching him for the first few days.
Do you have any idea how old he is? Mollies can live a few years under good conditions, balloon mollies only live about a year. Older mollies will have clamped fins most of the time and move slowly. They'll stay hidden or away from the other fish most of the time. I've lost a few fish to old age, so you might want to find out how old he was when you got him.
Pictures might help, if you have any, but with dalmatian or any spotted molly, it's hard to see some problems.
To treat him, I would remove him and place him in his own tank with brackish water and high heat (82-84) for a few days. Watch him closely and watch the other fish. If he's brought a sickness into the larger fish tank, the other fish might show the same signs. If this happens I would go with a full clean and add a little more salt then normal. If you see signs of an identifiable illness, like ich (white spots), velvet (gold coating) or fungus of any kind, these should be treated right away!
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