Use: angle(x1, y1, x2, y2) to get the angle between the 2 objects.
Then you can be even more fancy and use: distance(x1, y1, x2, y2) to calculate the distance between the objects, and depending on how far they are, you can make them wave less.
So you will need some form of collision detections first, then you can just loop through each grass sprite that is touched and use the above calculations.