Looks like mold growing on the kernels. There can be a few things that lead to moldy kernels, including walnut husk fly, a common pest of walnuts in Oregon. They tunnel into the kernel which allows for different fungi that cause mold to enter. Walnut blight (Xanthomonas arboricola pv. juglandis) can also lead to moldy kernels. Walnut blight infections that do not penetrate to the kernel nor result in nut drop, can create an entry for not only moth pests, but for the mold caused by fusarium and alternaria species. Monitor your crop early in the season for signs of walnut husk fly and walnut blight. You can find more information on these pests and management practices at the following links: https://extension.oregonstate.edu/catalog/pub/em-8907-growing-walnuts-oregon
and https://extension.oregonstate.edu/catalog/pub/ec-631-managing-diseases-insects-home-orchards