I’m going to guess that you’ve grown some mold on your basswood. Most likely it hasn’t moved very deep into the pores of the wood. Try letting it dry and sanding the surface. Maybe most of it will sand off. If some remains you can try a bleach solution and see if it that will do it with some light scrubbing with a toothbrush.
The real problem is when mold penetrates deep into the wood. I’ve not found anyway to remove the gray color left behind.