The bath may or may not work depending on what type of dye is used for the bark.

However, I would give the snake a lukewarm bath and then place him on newspaper or paper towel until his next shed to see if it goes away. I've never heard of a case of scale rot from substrate that is too dry, so that leads me to think it is the substrate. Even so, I'd keep a close eye on it and take him to the vet if it doesn't improve.