As your dough stores in the refrigerator, it might develop a uniform gray discoloration and liquid on its surface. This is not mold and can be safely ignored– it won’t affect the final baked result. You can just pour off the liquid and proceed with the recipe.
If you see patchy light or dark areas on your dough, whether smooth or fuzzy, that could be mold and the dough should be discarded. You are not likely to see mold if you follow our directions for maximum storage life, and keep the dough in the refrigerator.
If the dough has become hard and leathery, that suggests that there’s too much air-space in your container (or that it isn’t sealed well enough). You can decrease the effect of air that gets into the container by transferring into smaller containers as the dough is getting used up.
More in The New Artisan Bread in Five Minutes a Day, and our other books.