Mold in AC Closet

In our apartment we noticed in our AC closet that there is mold. We are not sure if this comes from the AC itself (AC is almost 15 years old) and changing the AC and the pipes going to the AC would fix that problem. Since all other areas are ok. We downloaded an app to see potential moisture but all looks good except for the AC closet and the closet behind the AC closet. I have attached pictures to it. We are not sure if just an AC replacement would do that or if other things need to be changed.