"What do I do about Mold on my Cutting Board?!"
Look, I hate those articles that drone on & on before getting to the solution so...
TL;DR: try lemon or lemon juice. Eh, maybe bleach, but I discourage it.
Recently, a customer contacted me about mold on their butcher block, and they weren't sure what to do. The piece was big enough that they couldn't just haul it in, and the site was far enough that I wouldn't be able to do a site visit. So what can you do besides find someone who is close enough? Well, do it yourself!
I want to get my least favorite way out of the way first. Bleach it. You could further dilute some bleach and apply that onto the board and scrub with sponge or a stainless steel scouring pad. That said, I am not crazy about putting bleach near my food. If there are alternatives, I'll happy try all of them before dropping the nuke that is bleach.
I suppose you could try salt, but that just seems like a hassle.
So what are my favorite methods? Lemon and Lemon Juice. You can repeat the application we went over with bleach--apply the lemon juice, let it sit for a few minutes, and scrub with a stainless steel scouring pad; however you can also scrub with an actual lemon. Just cut a fresh lemon in half and push the cut side into the board, releasing the citrus juice. Doing this and then scrubbing it should help a lot.
If these methods don't work, you could have us repair it, though it might make more sense to replace the one you have.