Nothing to worry about, its just allocated texture memory. With AGP cards it was called AGP aperture size and was controlled by the bios. With PCIe cards its controlled by the video driver and as far as I know cant be changed.
With AGP there is an option within your motherboard bios that allows you to select how much system memory can be used by the GPU. This is known as the AGP aperture size. You can choose various options (64, 128, 256 etc) but most people recommended you use one quarter the capacity of your system memory. With PCIe there is no such configuration left to the end user, it's all handled automatically by the video card driver. From what I can see, its decision is sensitive to the amount of system memory you have available and will at maximum only ever double the amount of available memory