AS2 dynamic mc _alpha property

programming flash
1

I am having problems with the _alpha property of dynamic masks. For some reason they don’t seem to accept it.
I have a holder mc that I use as a mask and the holder is full of dynamic mcs for which I would like to set apha properties. My code is:

_root.createEmptyMovieClip('holder',_root.getNextHighestDepth());
function init() {
 for (i=0; i<100; i++) {
  holder.attachMovie("msk","msk"+i,i,{_xscale:0, _yscale:0});
  _root.holder['msk'+i]._x = (550*Math.random());
  _root.holder['msk'+i]._y = (380*Math.random());
  moveThem(_root.holder['msk'+i]);
 }
}
init();
var rot:Number;
function moveThem(mc) {
 bg.setMask(holder);// sets the holder as the mask
 mc.rot = (20*Math.random());
 mc.onEnterFrame = function() {
  if (mc._xscale<50) {
   mc._xscale = mc._yscale += 1;
                                      mc._alpha -= 1;
  }
  this._rotation += mc.rot;
 };
}

The line

mc._alpha -= 1;

does not do anything. Am I missing something?