Try this.
Needs some tweaking but you get the idea. It's best to put the platform behavior in the family instead so you can mess with it via events. (Setting vector X will to 0 will let you move the object down only a pixel or two instead of ~6, to prevent that "jerk")
Also your sprite mask should never use collisions or behaviors; make a separate simple-shape object like a rectangle for that, then set the sprite mask to 0,0 of it every tick.