This is a new decorative element, sort of like cobwebs. Moss is the same as carpet (two pixels high). However, it can be walked through like a pressure plate.
Moss can grow upside down, on walls, on the ground, and has the same shape as carpet. It comes in five colours: Green, yellow, red, brown and blue. Each biome uses one of these moss colours (more detail below). Moss spawns naturally in caves, and rarely on exposed stone in some places (more detail below as well). Moss can be walked through like a pressure plate.
Moss spreads like grass, but only on stone, not andesite, diorite, etc. It can be placed on any block however, but won't grow unless the block is stone. Moss WILL NOT spread onto ores, so you will still be able to see them. Moss will be fully opaque and will be washed away by water. It can be collected with shears and placed on any block that isn't transparent.
MOSS COLOURS AND BIOMES:
Green Moss: Forest, jungle
Yellow Moss: Extreme hills, taiga
Red Moss: Mesa, (maybe)desert
Brown Moss: Plains, savannah
Blue Moss: Oceans, rivers, tundra
Weeds: This idea isn't going to happen, feel free to recommend stuff for it in a spoiler.
Weeds are plants that can grow over growing crops (e.g: if you are growing carrots, there might be a 1/12 chance of a weed growing throughout the entire time.) Weeds can be killed with a "weed whacker" (recipe below) by left clicking on the crop. Weed whackers cannot kill crops, only weeds. Weeds will grow at a rapid speed (In around a day), and then proceed to spread more weeds on still growing crops. Weeds have three distinct stages. Each takes around 4 minutes in real life. The first shows a small weed in the centre of the crop. The second has the weed much larger, but weed whacking it will still let the crop survive. The third one has the original crop removed, and killing it leaves dirt behind. Then there is the germination stage. When weeds have germinated more than a day ago, they become dead bushes, turning the farmland underneath them into coarse dirt. Weeds, when dead as bushes, have a 1/3 chance of dropping "weed seeds" which can be planted on farmland, dirt or coarse dirt, or even to grief other people's farms!
Additionally, dead weeds have a 1 in 10 chance of dropping an 'exotic fruit,' a special fruit which has multiple effects.
Let me know what you think. Give suggestions and comments please!
Weeds would be annoying. I wouldn't want to go to my farm to find that it eventually became a bunch of dead bushes, even if the chance is low. You even listed it as a griefing material, which is another reason why I don't support weeds.
I support moss however, because it has an aesthetic use.
I'd just like to point out that mossy cobble is already renewable, as of 1.8 it's craftable with cobble and vines, vines can be grown on any block and harvested with shears. Cobble can be generated with lava and water.
The idea seems nice, it would be good to have something to decorate ceilings without changing the block.