White and Grey are not the same color. A grey horse is born a dark color and has mostly(excluding skin under white markings) dark colored skin. As the horse gets older he turns lighter. Some horses stay different shades of grey and some turn almost white all over but they are still classified as a grey because that is what they were born. A white horse on the other hand is born snow white and has light/pink colored skin. It is easy to tell the difference if you look at the horse's nose. The nose of a grey horse will look outlined in black, while the nose of a white horse will look almost pink.
There also can not be an albino horse. There can be albino snakes, rabbits, mice, and even people but the gene for albino doesn't exist in the equine species. All white horses you see are just that....white, not albino.