Every time that has happened to me it has been mold. Granted, only two times and they had been in secondary's that had their airlocks dried out, but that is my experience. If this brew is dear to you, transfer it to a clinically-clean fermentation vessel and campden it if the gravity is low enough. White mold *in general* isn't poisonous (think dry aged sausage), and low pH/alcoholic environments generally prohibit dangerous organisms from proliferating, but that doesn't mean it won't change the flavor.