It would help to see the actual design and we also need to know which version of AE...
And since hair rarely moves of its own accord, it would also help to know how this hair will be moved. A slow turn of the head? Blown by a gentle breeze? A brush moving through it?
And how realistic-looking do you expect this movement to be?
If this is a simple silhouette I would probably do one of three things.
1. Put the hair on a separate layer, use Fractal Noise to create an animated gradient, Pre-compose the gradient, and then add Displacement Map to the hair layer.
2. Put the hair on a separate layer and use Luquify or mesh warp to animate.
3. Put the hair on a separate layer and use Puppet Pin
There are dozens of other ways to do this. The easiest depends on you design.