Moisten the area with water with a rag or paint brush, It helps the spackle adhere better. Apply spackle to the surface hole and cover with drywall paper or nylon mesh, integrating it into the spackle. Smooth out, let dry, sand. Don't try to fill all of a large crack or hole all at once. Repeat spackling and sanding between coats.
Mildew can be a real predicament in baths, kitchen, laundry rooms, damp closets and basements.
Remove mildew with a solution of half household laundry bleach and water. Wear rubber gloves and goggles. Several good scrubbing of bleach and water should remove all the mildew. Let the area dry thoroughly. You may consider having a mildew inhibitor added to your paint and or getting better ventilation to the area.