The floor of this shelter has been replaced with moss stone from a dungeon, for aesthetic purposes.
Moss Stone (previously known as Mossy Cobblestone) was first introduced in Survival Test and remains in all later versions of the game. It looks like Cobblestone, but with moss growing on it. Like Cobblestone, on the upper left, there is an 'L' that points to the north and east. But unlike standard Cobblestone, Moss Stone cannot be smelted or crafted into anything. The only use is for construction - often used in combination with normal Cobblestone for decoration purposes, such as to make buildings look older or to make Nether portals more eerie. It is currently only found naturally as collections of 30-64 at a time in underground Dungeons, Abandoned Mine Shafts, and NPC Villages.
Moss Stone appears very similarly to Netherrack, following a similar pattern, but with different color schemes. Moss Stone being grey with green moss and Netherrack being bright-red with dark-red areas.
History
In Beta 1.8 Moss Stone's texture changed from File:Moss Stone-Pre Beta 1.8.png, to its current texture.