Hi and welcome.
We're gonna need some more info before a diagnosis is possible.
Ok,
whats the size of your tank?
what fish, and how many of each have you got in there?
what are your water readings (ammonia, etc)
whats your tank maintainance regime like (cleaning, feeding etc)
although I don't really think many of the above will have much to do with whats going on, it will help.
My gut feeling (although I don't know the size of the tank) is a terratorial issue - although I'm not too hot on molly's and I didn't think that was usually an issue.............