Mold from A/C in apt?

Last few days I thought I was smelling Mold in my home. I just changed the filter today and when I removed the old one I def. smell mold inside the opening. I am only living here 6 months and did have an inspection done and ducts cleaned in December when I was told the ducts look great...no signs of mold or accumulated dust...but now this smell is quite evident. I need to let my landlord know...is it a huge problem or could it be something normal for this time of year. Im on a ground level apt in Charlotte with a crawl space below. Thanks