Snow is both a mineral and a rock. Look at the definition of a mineral: A naturally occurring, inorganic solid with a definite chemical formula, and an internal crystalline structure. Snow is natural. It is inorganic, which means it is not alive and has never been alive. It is solid, not a liquid or a gas. If you doubt that, let someone hit you with a snowball. It is definitely solid. It has a chemical formula, H2O, and the shape of the snowflake gives us a look at its crystalline structure. Snow is a mineral.
A rock is a naturally occurring solid which forms mappable layers in the Earth. This is a much broader definition. We have already seen that snow is natural and solid. Looking out my window, I can see a large, mappable layer of snow. It is a rock. What kind of rock is it? It is deposited by the air, which tells us that it is a sedimentary rock.