Monster stop his movement after he reaches player destination

Hello,

If player leaves from sphere radius, monster will reach last distance where player leaves and he will stay in his position.

Can you guys give me some advice what I need to do ?

There is my script :

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.AI;
 
 [RequireComponent(typeof(SphereCollider))]
 public class Monster : MonoBehaviour    {
     
     GameObject player;
     bool playerInSphere;
     public float distance = 1.0f;
     public Transform target;
     public float speed = 3f;
 
     // Use this for initialization
     void Start ()
     {
                 
     }
 
     // Update is called once per frame
     void Update ()
     {
         if (playerInSphere == true)
         {
             MoveToPlayer ();
         }
     }
 
     void OnTriggerEnter (Collider other)
     {
         if (other.gameObject.tag == "Player") 
         {
             Debug.Log ("He entered");
             playerInSphere = true;
         }
     }
 
     void OnTriggerExit (Collider other)
     {
         if (other.gameObject.tag == "Player") 
         {
             StopMovement ();
             Debug.Log ("He leaves");
         }
     }
 
     public void MoveToPlayer ()
     {
         //rotate to look at player
         transform.LookAt (target.position);
         transform.Rotate (new Vector3 (0, -90, 0), Space.Self);
 
         //move towards player
         if (Vector3.Distance (transform.position, target.position) > distance) 
         {
             transform.Translate (new Vector3 (speed * Time.deltaTime, 0, 0));
         }
     }
 
     public void StopMovement ()
     {
         
     }
 
 }