First thing, Black Mollies are picky when it comes to ich and fungus. They like salt in their water and pristine water conditions to really thrive. If it is fungus, which it appears to be, then you will want to add some salt according to instructions to help him rebuild his slime coat and heal. Salt plus Mollies is a winning combo. Its not ich as its too big of a spot. There is also the small chance he scraped his head on something and has a wound now.