If you're getting mold under the wax, there will be a small opening - perhaps just a "pinhole" somewhere. Be sure when you're waxing that it is fully covered. You'll need to look it over carefully after waxing and any areas that appear thin - or questionable in any way that there may be a hole, just touch it up with a bit of wax. (Of course you need to be sure there's no mold on it when you wax it in the first place.)
That should keep it from molding underneith.