Could there be a hidden source of moisture in your house? That is what I would be suspecting if mold is growing. The hidden moisture may be allowing the humidity to rise high enough that mold will start growing. You could try hepa vacuming all items in your house but the spores may be embedded within your porus items. How is your health? Do you have any unusual health problems? Have you had anywater leaks that have not been fixed within 12 t 48 hours? That is all it takes for mold to start growing within your roof.