Rastejam comes from the verb "rastejar," meaning "to crawl" or "to grovel." It's a vivid, somewhat harsh verb that conjures the image of something moving low to the ground, often with a sense of humiliation.
In the song, the line "Quase não consigo acreditar no quanto ainda rastejam" ("I can hardly believe how much they still crawl") uses this strong image to express disbelief at how much certain feelings still persist, almost shamefully, after everything.
"Face e Avesso" means "the front and the back" of something, and that is exactly what Vanessa da Mata is doing here: showing the ugly reverse side of a society that likes to talk about morality.
She opens by saying she knows the difference between how people present themselves and how they actually act. The song then lists who pays the price for that gap: women who carry the blame for other people's guilt, the poor who are looked at like scenery by the rich, and children and indigenous communities left to bear the damage.
The "ahã ahã" refrain that closes the song isn't really words. It's closer to a sigh, an exhausted sound after naming all of it, and it gives the song its resigned, matter-of-fact tone rather than an angry one.