low humidity for a couple days

I occasionally have to go home from school on weekends, and without me here to mist the tank the humidity will drop into the 40's. will my ball be ok if its in this humidity for a couple days?

As long as the snake isn't in shed it shouldn't be a problem. If the snake is in shed the humidity really needs to stay 60% or higher.

Ok cool, I was even thinking of putting some damp moss in one of the hides before I leave

there is no problem with the a BP being at 40% Rh for extended periods as long as they have access to fresh water it is fine but for a shed. A humid hide can be added during a shed as an aid this is often all is needed.