Those tiny specs you see on your siding are called “artillery fungus” or “shotgun fungus”. They come from the bark mulch that you have decorating the exterior of your home. When you buy mulch, it is important to find mulch that does not have bark as an ingredient. The fungus is already present in the mulch when purchased. This fungus will shoot up from the mulch and attach itself to the siding, and is extremely difficult to remove after it has attached itself to the siding.