There is Mildew on the Door Gasket
There is mold or foreign object on the gasket.
When opening/closing the door, the cool air from inside and warm air from outside contact each other and create moisture. If it is not dehydrated, there might be mold.
As you use the refrigerator, the gasket can get dirty due to dust and stuff.
How to fix
Clean the dirt or mold with detergent and wipe with a dry cloth.