Most of the transparent pixels stay the same, and the transparent pixel clusters do not appear to move, which hurts the believability of it. Just simply turning them green here and there won't help though -- the transparent clusters need to relocate periodically, which, if you're using Graphics Gale to do this, you can drag and move areas of the image by right-clicking and selecting a rectangle and then clicking and dragging on it to shift them around. It would create the illusion of those transparent 'clumps' moving around.
Outside of that, your animation looks very believable.