Not mold - just advanced nitrogen deficiency. if something is isolated, it's okay to ignore. If it progresses, you need to react to it once the symptoms and other cross-referenced information leads to a confident answer. In this case, N-deficiency is one of the easiest things to diagnose. leaf symptoms are indiscrete. if you only rely on them, you make mistakes at some point. image search for a 'marijuana leaf symptom chart' - keep asking for help as you need but start looking it over... you'll soon be as good as anyone else can be at guessing what the cause is.