How to get Materia from Aeris' hair in the Forgotten Capital?

Solution 1:

That is the White Materia, used for Holy, the most powerful White magic in the game.

It plays an important role in the plot so the plot summary is in the spoiler below. If you want more spoilers, the plot related to it can be hound here on the FF wiki

The White Materia is used to summon Holy. This is the only power capable of stopping Meteor, which was summoned by Sephiroth using the Black Materia. Aerith goes to the Forgotten Capital to pray to Holy, but is killed by Sephiroth. It turns out that she did successfully summon Holy, but Sephiroth is preventing it taking effect. You kill Sephiroth, Holy emerges, but Meteor is too close to be stopped by Holy alone. The Lifestream is used to push Meteor back, allowing Holy to destroy it.

Solution 2:

It is the White Materia. If you recall, when you first met Aeris, she said that she had a materia that was good for absolutely nothing (ironic, eh?). This is that materia and no, there is no way to get it for yourself. Though it does come back into the game later on in a way (and you still don't get to have it then, but that is okay).