Is it that big white area on the molly? If so, Looks like fungus to me.
Need my eyes testing the fluffy white bit I saw was on some wood.
The white on the fish is it fluffy or bleached out.
Any signs of flicking and rubbing.
The symtoms you are describing can be parasite to bacterial.
I think I would take the bacterial route for now if the fish isn't flicking.
Water stats would be good.
Mollies do better it brackish water, but it depends on what other type of fish you keep.
I agree with kpintheshed. Mollies need salt or really clean water, or else they'll get ich, fungus, ect....
Do you know what caused the wound in the first place? Or did it just kind appear?
When you get your water stats, you should note that in freshwater mollies need very low nitrates compared to other fish, also check your pH, since corys and mollies have pretty much exact opposite preferences (though corys are more adaptable than mollies). Also, not really the issue here, but corys should be kept in groups .
How long have you had mollies in that tank?
The suggested bacterial medication should be tried before anything for parasites....
Some parasite medications could just worsen the situation .