You should have the following command in a command block on clock of some sort or a repeating command block if you are in 1.9:
Testforblock x y z wool 5
Replace the X,Y, and Z with the coordinates of the block. Because colored wool doesn't have different names for each color, you must use the damage value to testfor the color. The minecraft wiki page on damage values shows that green is value 5 for wool. When it doesn't detect it, the comparator will turn off.
Note: if you put -1 as the damage value it will do it no matter what damage value the block is
Alternatively, this command can be used but the comparator will turn on when it breaks:
Testforblock x y z air 0
Air doesn't have data values so 0 or -1 is all you need.