Yep, those nuts need to dry out and cure out before they are good for eatin'. When we were homesteading, we had several walnut trees in the yard and it was one of my jobs to step on the rotten hulls and remove the nut, gathering them for drying. My shoes and hands were always stained at that time of year, no matter what gloves we wore....hard on a kid to be the only one showing up at school with hands that won't wash "clean'.
I remember that one year we hauled them off to a place that bought the walnuts...had a big machine that hulled the nuts and all.