There's often no way of knowing what one has done 'wrong'. While it might be little late if fungii are actively growing, a drop or two of hydrogen peroxide does inhibit its growth. On established cuttings, that is, those already rooted, too high humidity can cause mildew to take them out. Opening the bag, as you have done, is the solution (though mine are grown in pots with plastic bottles and caps which I can remove).
Steve.