It's probably just dirt. It will come off during shed or just come off itself some other way. Can I ask though why you were Soaking your snake yesterday if he is in shed? Soaking prior to a shed will remove the oils naturally secreted by the snake that helps him shed. Soaking should be a last resort option for sheds that got stuck on AFTER he tried to shed himself. Also, IF a soak is needed you want the water to be only as high as a quarter of his body height (as he is laying flat). Not trying to get down on you, just giving you some advice.