Pour bleach and warm water into a bucket. The bucket should contain nine parts water and one part bleach.
Wipe the solution with a rag on the moldy areas of the case. Because the bleach is such a small part of the solution, blotching shouldn't be a problem but, to make sure, test the solution on a small area of the fabric.
Place the case in the sun until dry.
Wipe the once-moldy areas of the inside of the case with a disinfecting wipe to further disinfect the case and dry again.
Vacuum the inside of the guitar case to remove harmful dust and particles from the surface of the fabric. Use a vacuum with a long, skinny nozzle to get into the hard-to-reach corners of the case.
Remove the smell of the bleach by spraying with Fabreze air freshener and/or put a bag of coffee grounds into the case overnight. Repeat until smell is gone.