[Aquarian]

3% hydrogen peroxide, simple and cheap. Add a little boric acid and you will help prevent it from coming back. If you add boric acid, try not to get it on your skin - it can cause issues if over exposed. Bleach, borax, and most other mold removers break down into this or that and water and SALT. Where there is salt there will be moisture and moisture breeds mold.

 

[Aquarian]

I should have also mentioned that while peroxide will kill mold, it will not remove mold stains.

 

[Aquarian]

3% hydrogen peroxide, simple and cheap. Add a little boric acid and you will help prevent it from coming back. If you add boric acid, try not to get it on your skin - it can cause issues if over exposed. Bleach, borax, and most other mold removers break down into this or that and water and SALT. Where there is salt there will be moisture and moisture breeds mold.

Borax and boric acid don't break down into anything. Think for a moment just how long the borax deposits have been exposed the elements. A borax film with prevent mold until it has been washed away or rubbed off (I've done the testing), regardless of humidity... but dry is always better because it will rub off eventually.

You might want to give this a read. Borax is not pure boron.
https://www.boricacid.net.au/six-reasons-why-boric-acid-is-better-than-borax/