[Cabey4]

I would:
1. run a very small median blur
2. Very slight HSV filtering - barley anything - remove stuff that's say bright green. You will have much more than just a ball
3. Binary mask or something so that you have the full image with parts that didn't get through HSV now black - as opposed to just a black and white image that you would otherwise have

Now, it would make it a lot easier to paint or colour the parts of the robot that border the boulder, from the camera's perspective. Enough that the HSV filtering will remove it, so you should end up with the gray ball (or part of it) not touching the robot.

4. Edge detection
5. filter based on criteria

You could also use HoughCircles, but it still might go crazy without the boulder.