To be honest, to me, it looks more like a fungus of some sort. Do any other fish have tge white growths on them?
There are several over the counter treatments like Melafix, fin and body cure, fungus cure, and others. But really the root cause of most problems is water quality. What are your parameters reading as?
Stress can indeed be a factor.
As a complete shot in the dark based on what you said something like this could have happened:
The molly nipped at this guy and caused some damage.
The water quality not being as good as it could be caused the wounds to not heal.
The wounds not healing in bad water ended up growing a fungus.
Do you have hiding places for your fish to get away from each other? That might help with the nipping.