I really can't believe it's mold....more than likely it's a spot where water was left standing.
I can't tell you how to get rid of it without knowing what is on the board as far as finish.
Steel wool likely will treat it.
And then make VERY sure the board is dry, after you use it and wipe it lay it upside down over night so there is no chance of water collecting in that groove.
Linda C

Thanks Linda! I found a site on proper cleaning of wood cutting boards. I read that it is good to rub the spot with a coarse salt (Kosher) and lemon juice. I tried it and it did make an improvement. I also read that vinegar and clorox bleach will work.

Wouldn't want a clorox smell on my cutting board......