Moss stone should be renewable, to prevent pickaxes from being worn out if the player collects moss stone a spawner or jungle ruin. In order for Moss Stone to be farmed, the player should place one block of moss stone in a 3x3 square of cobblestone (if there is a larger square, every moss stone block will spread to the eight blocks around it, and so on.) The same should apply for moss stone brick and mossy stone walls. The moss should spread up walls, as well as on the ground and on ceilings.
I think that it should be farmable, but not in the way you're suggesting. If it worked in the way you suggest, then you couldn't build anything with both mossy cobble and normal cobble, because the normal cobble would just become mossy.
Maybe you could move the formula onto a crafting table, with a perimeter of 8 cobblestone/stone blocks around 1 mossy stone/stone block returning 9 mossy stone/stone blocks.
Or how about if there are vines on a cobblestone block, a pickaxe with silk touch will yield a mossy stone provided the vine is still attached to the cobblestone when it breaks.
Moss Stone Farming.