Hello!
So uhm, title says it all really.
This is what is happening:
This is how the material is setup:
Any help would be appreciated, thanks! 
Try building the lighting in your project
switch back to place mode (the box and lightbulb in the top left) and click on that section of terrain. Make sure that it has the same material in the details panel as the green sections do. If it does, then try repainting it all completely to your stone layer first, then try painting the grass on