One thing to watch out for: It looks as though you are picking up some kind of darker substance and it's being transferred to the Sculpey.
I admit, back in the day when I smoked cigarettes, my sculptures always had darker grey/black lines randomly all over from picking up ash on my fingers. The lines on your work resemble them very closely, so I offer my opinion here.
It doesn't ruin the sculpture or anything, but best to keep it as clean as possible.