If it is salt, you can hit it with some phosphoric wheel cleaner and a plastic brush and see it that removes it. It might take a little time for the acid to dissolve it. Brush on the phosphoric, let it sit for five to ten minutes and then wash it off with a power water spray. If it is mold, you can attack it with some mold remover and a brush, letting it sit again for a little while to dissolve the mold. I think it is salt, because mold doesn't survive that easily in the open air. The color and crystals seem to be salt.