How to animate a enemy when he is following

i want to the enemy(Zombie) plays an animation when following the player, i already have something but it doesnt work using UnityEngine; using System.Collections;

 [RequireComponent (typeof (CharacterController))]
 public class BehaviorZombie : MonoBehaviour {
 
     //Component
     public Transform target;
 
     //Movement
     public float moveSpeed = 5;
 
     //States
     private int range = 5;
     private int state = 0;
 
 
 
 
     void Start () {
     
         target = GameObject.FindGameObjectWithTag("Player").transform;
         animation.Play("Idle", PlayMode.StopAll);
         
     }
     
 
     void Update () {
 
         if (Vector3.Distance(transform.position, target.position) < range){
             IdleState();
         }
 
 
         if (Vector3.Distance(transform.position, target.position) < range){
             AlertState();
         }
 
     
     }
 
     private void IdleState () {
 
 
     }
     private void AlertState () {
         Debug.Log("following");
         transform.LookAt(target);
         transform.Translate(Vector3.forward * Time.deltaTime * moveSpeed);
         animation.Play("Walk_Zombie", PlayMode.StopAll);
     }
 
 }

i want them to blend between an idle animation, and a walk animation...

thanks for your time excuse my english