Fluids is the correct technical term. Magma is a complex mix of elements, and as it moves through the crust it cools. As it cools, the mix reaches critical temperatures where portions of the mix no longer can remain mixed, and so they separate out. It's still basically molten rock, but one fluid separates from another fluid or from a crystallizing mass. The separated fluid may be lighter or more fluid, likely contains water or carbon. But we can't tell specifics, as this is happening deep underground.