I would be very reluctant to do this mold removal with bleach. It may leave some crystals, after drying,in the cloth and harm the guns in the future.
The vacuuming will probably work and a light sponging with clear water.
If there be an odor, put some dry coffee grounds in and close the case for a week. It will be absorbed and smell good again.
Consult a dry cleaner, that knows his stuff, if the above fails.

Best,
John

Humble member of the League of Extraodinary Gentlemen (LEG). Joined 14 March, 2006. Member #1.