How to tell apart mold and flour on sourdough bread?
Scrape at it with your fingernail. Flour will come off in a... floury manner, revealing the cranny it was stuck in. Mold won't be limited to the crannies. Also, it'll appear on the cut surface before it appears on the crust. Additionally, it'll be different from the color of the flour (in the same area of the loaf).

FWIW, that's definitely flour on most of the bottom, though I do see a couple of lighter spots that could be mold.