Mine has a brown mark as well, like a birth mark. It is under it's mouth. Darker than yours. Not sure it's an illness - check the molly's behaviour - is it acting weird, e.g. gasping for air, inactive, not eating normally. I'm not an expert but proper diagnosis is needed.
It may just be an injury from colliding with another fish or with decor. I agree completely with arunallen93's helpful advice - if it's acting normally, I wouldn't worry (yet).
If it's behaving differently from usual or not eating, then let us know - and also let us know what ammonia, nitrite, nitrates, PH and water hardness is, using a liquid-based test kit, as well as how long you've had the molly, other tankmates and how long you've had them, and size of tank. This information will help us diagnose further if required.