Horses are fight or flight animals - And if they can, they will choose flight 99% of the time as they aren't equipped well to fight.
If the filly is nervous, asking her to stop moving her feet will nly increase the feeling of being 'trapped' - Unable to flee from any perceived danger. That's why horses who are scared/nervous often prance.
You will have a hard time convincing her to stand still while she is still worried. I would drift more toward gettingher comfortable within the environment before asking her to stand there. It makes it trickier that she is so young - normally I would just work a horse within the scary environment, and work them hard. Work them until they are either focussed on me and not the nerves, or if that doesn't work, work them until they are so tired they can't pay attention to the nerves. Then I would stop, stand for a few minutes, and leave the area.