First of all, as @a59303 already said, the subdiv modifier should be above (this only increases the resolution but doesn’t solve the stretching).
The stretching is caused by the local coordinates. Because in this case the modifier takes the x and y coordinates of each vertex and uses these as the u and v coordinates of the image. So in places where the x,y values are nearly the same and only the z coordinate changes (vertical parts of stone) there will be nearly the same pixels → looks stretched.
@a59303 suggested UV-Unwrapping and then using the UV-Map. This will only solve the problem partly. Because you can’t perfectly unwrap a sphere, ther will always be a visible seam.
I would suggest to either use multiple displacement modifiers with different 3D Noises on top of each other. Or to create the displacement procedurally with shader nodes and in the end bake it to a UV-Map to use it in a displacement modifier.