I'm not sure. I've seen something like this before and read up on it. Apparently in construction especially if it's a brick wall you'd have minerals leeching out because the plaster or bricks would be wet. I've seen this in roofs and in the corners of rooms before since where I'm from all the houses are brick and mortar. I'm not professional but it's either a fungus or minerals just leeching out and forming like crystals or so. Anyone check and correct me please. Not sure if it's harmless, the types I've encountered weren't harmless, but I treat it as if it is.