So you were in your basement the other day, and in a mood to worry about things, and you looked up at the subfloor (the six-inch boards that most often run diagonally and support your floors above) and you thought, "What's that greyish powdery substance on those boards?" Move on, dear worrier, it's not grey mold.
In most cities, at the beginning of the twentieth century, there were no cement mixers or Portland Cement. There was a guy, building a house, using the things he had on site. So most often, those master carpenters took the boards that now support your floors, and made a frame with them. They mixed up some of the rocks on site and made a homemade cement. Then they poured it into that mold to create the concrete walls you're now checking out. Once the concrete set, they took apart the frame and reused the wood as your sub-floor.
These builders were recycling before recycling was cool.
So that grey dust that you see? It's the ghost of the concrete that now supports your basement walls. Cool huh? No need to worry about the grey mold. (At least for now!)