What vehicle? Just curious.
The change intervals can vary as to where the vehicle is driven. Dusty western USA locations would need to be changed more often than in a "cleaner" city environment, for example. What we don't know is what micron rating these filters usually have.
Did the possible mold spots disappear when sprayed with a hydrogen peroxide solution or Lysol? Mold can exist, unseen, inside the hvac system's a/c evaporator core. Especially if the condensate drain might be plugged up!
Many years ago, I was at a friend's service station. He had a customer's car on the lube rack. I walked over and was looking under it. It appeared that the condensate drain's rubber end was stuck shut. I touched it enough to break it open a bit. First a drop or two, then a "flood" of moldy, smelly water (about a quart!) gushed out. That was in a 1980s car.
There are some products to treat and clean the evaporator cores to remove such mold and "odors".
Enjoy!
CBODY67