FIrst of all, welcome to the forum.
Second of all...oh, dear. You should never, never, never let aquarium fish go into wild bodies of water. It might survive through the summer, but almost certainly will die by freezing come winter. And if it does somehow survive and, heaven forbid, find another molly with whom to breed, it could definitely hurt the lake ecosystem. Even if it doesn't, there is the possibility of it introducing pathogens for which the local fish have no resistance, though I would guess that, in an area as populated as Iowa, the local fish have probably seen it all.
What's done is done. You aren't the first to do it. Please don't do it again. For future reference, it is usually possible to find a new home for unwanted fish, and failing that, there are humane ways of euthanizing one.