How can I clean decomposing grass goo off my concrete driveway?
As the grass decompose, it releases tannin, a colorful byproduct of the natural decaying process. While the color will eventually fade from concrete, you can help hurry the process along with a little bit of driveway cleaning.

Spray the stained area with a garden hose to rinse away dirt. In a large plastic bucket, mix ½ cup of liquid dish detergent into 3 gallons of hot water. (Or, for the best results on tannin stains, use a powdered laundry detergent that’s advertised as being good at removing food stains.) Slowly pour the solution onto the tannin stains and work the liquid into the concrete using a stiff-bristle nylon brush. Rinse with the garden hose and, while the concrete is still wet, sprinkle powdered laundry detergent over the stains and scrub again. Leave the detergent on for a few minutes then rinse away. Repeat if necessary