It is usually sold in small bags and is used for craft projects. Sphagnum moss is different from sphagnum peat moss. Sphagnum moss is the gray stringy stuff used in craft or floral projects. It is the moss before it decomposes to become peat moss. Greensand is also known as glauconite.
Greensand is the pulverized rock of a sandy clay material called iron potassium silicate.
It contains 6-7 percent potash and contains up to 30 trace minerals, magnesium and silica. Potassium is important for disease resistance and winter hardiness. I've often seen greensand recommended for roses as a soil conditioner and fertilizer. If you happen to find all this fascinating or at least mildly interesting, you may want to consider the Master Gardener program. Training starts in January so don't delay.