You could simplify the mouse over script, and just control the healt bar visibility using guiText.material.color.a:
EDITED: Now that I'm completely awake, these scripts are correct and working. The idea is to make the health bar disappear by reducing its alpha to 0. When the mouse pointer is over any enemy, it recharges the health bar alpha (variable show) to 1 each frame, so the bar appears solid over the enemy. When the mouse isn't over the enemy anymore, the show variable is decremented to zero in duration seconds, making the bar vanish nicely.
This is the EnemyHealth1.js script:

var hp:float = 100; // health value, 0..maxHp
var maxHp:float = 100; 
var healthBarWidth:int = 20;
var myHealthBar:GameObject;
private var myHb:GameObject;
var posX:float;
var posY:float;
var show: float = 0;       // health bar appears when show > 0
var duration: float = 0.6; // health bar vanishes for duration seconds

function Start () {
  myHb = Instantiate(myHealthBar, transform.position, transform.rotation);
}

function Update () {
  if (show>=0){
    myHb.guiTexture.color.a = show; // variable show controls the bar alpha
    show -= Time.deltaTime/duration; // fades out health bar unless show is recharged
    myHb.transform.position = Camera.main.WorldToViewportPoint(transform.position);
    myHb.transform.position.x -= posX;
    myHb.transform.position.y -= posY;
    myHb.transform.localScale = Vector3.zero; // <- this does nothing
    var healthPercent:float = Mathf.Clamp(hp / maxHp, 0, 1);
    myHb.guiTexture.pixelInset = Rect (10, 10, healthBarWidth * healthPercent, 5);
  }
}

This is the mouse over script:

 function Update () {  var ray = Camera.main.ScreenPointToRay (Input.mousePosition);  var hit : RaycastHit;  if(Physics.Raycast(ray, hit, 10))  {  if(hit.collider.tag == "Enemy")  { // while mouse over enemy recharges variable show  hit.collider.GetComponent(EnemyHealth1).show = 1;  }  } } 

I'm seeing that the main difference between what you are doing and what I would do is that you are using a physical GUITexture when I would just use GUI.Box to create it by hand.

I would

• Use two scripts like you do, but have the enemy script be nothing but a hold it's health.
• In the first script, Raycast test to find and acquire the current health of the enemy. Just like you do in your 2nd script right now.
• Position your health bar AT THE CURSOR (or slightly offset to make it look nice)
• Then create a GUI.Box or GUI.DrawTexture or what ever makes your health look nice.
• Creating a GUI by hand has been, in my experience, easier and more reliable. I don't trust code I can't see, so I prefer this. It seems to work for me.

Also, you are calling GetComponent Waaaaaaaaaaaaaaaaaaaaay too often. It's an expensive operation, and the output of it doesn't change unless you do something crazy, so just call it once and store it in a var.

This seems to work for me, but it might not for you. I find that by reducing the number of things you have to worry about (example: WorldToScreenPoint) it will make things a lot easier for you in the long run.