Highlighting all clones instead of the one the mouse is over.

I have an empty GameObject called IGController with a script that instantiates a prefab into a 9x9 grid. On the prefab is another script that is supposed to highlight one grid object when moused over. Instead it highlights all of them. How can I get it to only highlight the one my mouse is over?

I know there are plenty of Unity questions like this, but I couldn't seem to get it working even after reading them..

---------- Script on IGController: ----------

 using UnityEngine;
 using System.Collections;
 
 public class IGController : MonoBehaviour {
 
     public GameObject trackPiece;
     public int gridSpacing = 10;
     public int gridWidth = 9;
     public int gridHeight = 9;    
     
 
     void Awake () {
         GameObject [,] grid = new GameObject [gridWidth, gridHeight];
         for (int x = 0; x < gridWidth; x++) {
             for (int z = 0; z < gridHeight; z++) {
                 GameObject gridPlane = (GameObject) Instantiate (trackPiece);
                 gridPlane.transform.position = new Vector3(gridPlane.transform.position.x + (x * gridSpacing), gridPlane.transform.position.y, gridPlane.transform.position.z + (z * gridSpacing));
                 grid [x,z] = gridPlane;
             }
         }
     }
 }

---------- Script on prefab: ----------

 using UnityEngine;
 using System.Collections;
 
 public class Highlight : MonoBehaviour {
 
     public Renderer rend;
     public Material normalColor;
     public Color highlightColor;
 
 
     void Start () {
         rend = this.GetComponent<Renderer>();
         rend.material = normalColor;        
     }
     
     void Update () {
         Ray ray = Camera.main.ScreenPointToRay (Input.mousePosition);
         RaycastHit hitInfo;
         if (Physics.Raycast (ray, out hitInfo, Mathf.Infinity)) {
             rend.material.color = highlightColor;
         }
         else {
             rend.material = normalColor;
         }
     }
 }