Ahab notices bits of green moss growing in the tiny cracks of the mast. what is suggested in his observation that the old mast bears evidence of new life but that his own old head does not?

As he notices the green moss growing in the cracks of the mast and he is told that everything that is part of the nature will still continue even though something severe happens to someone. This is the reason he did not change the obsession he had even though he is going to die because of it.