One of my Mollies has recently started swimming along the water surface and splashing the water out with it's tail fin. It's really quite random. Just the Molly playing around? I can't see any signs of disease on it.
It doesn't sound good. IME any different behaviour from the mollies available from LFSs is generally followed by them dying in a few days. Personally I avoid them and would get platies if you really want a livebearer or add salt to their water which seems to help a lot.