Mossy Cobblestone is a decorative block, added in 0.8.0. Its texture is similar to Cobblestone, but with moss on it. Its function is exactly the same as cobblestone. Mossy Cobblestone could not be obtained in Survival until 0.9.0, although it was added into Creative mode before that.
6 Mossy Cobblestone in a Stonecutter = 6 Mossy Cobblestone Wall