Something like this?
https://www.dropbox.com/s/byq48wyr82f6v ... .capx?dl=0
Here's the step-by-step:
1. Put your bg in a layer.
2. Create another layer over it (mask layer) and set the following layer attributes:
Force own texture: Yes
3. Create the "eye" sprite (it may have any shape you want) on the mask layer and set Effects>Blend Mode: Destination out.
4. Move the sprite around using your favorit method. In this example I gave the "eye" sprite a Bullet behavior and added a "Every x seconds" event that randomly chooses the Sprite.Bullet.AngleOfMotion among 360 degrees.
Hope it helps. Cheers!
Thanks brunopalermo , for coming back to me, however I do not seem to be able to download your capx as it states that it is not there.
** EDIT **
No need for the capx file now, I just followed your instructions and it looks good ! thanks again, just need to get the behaviour of the moving sprite to work a bit better now.