You may ask this question if a gray play button is showed by default in your Android WebView.
Similar questions are:
This happens because this is the default image, included as a poster, displayed in Android WebView before playback start.
You may solve the problem adding the following snippet to a custom WebViewClient:
@Override
public Bitmap getDefaultVideoPoster() {
return Bitmap.createBitmap(10, 10, Bitmap.Config.ARGB_8888);
}