In Sara's time, the primary concern of the Resistance was the aftermath of Bliss Day, still fresh in thier minds. The aliens exploited this by taking bizzare, cold, mechanical forms. While Sara tends to resort to vauge generalities/euphemism, Joshua's vivid descriptions of titanium giants made out of infinitely fractal faceless people haunts Archer, Hope and Casey still, and Jon to a lesser extent remembers scattered details.
(This is, of course, what they did when they weren't flooding Joshua's mind with hallucinations of Jim Preston disowning him, or Anna's parents attacking Meredith in a homophobic rage, or a tall, dark, handsome incubus seducing the life out of newly pubescent Sara, or... Somehow, that sort of attack never made it into Josh's war or ghost stories.)
But Bliss Day has faded from the minds of the second generation, except as a mandatory history lesson. More to the point, the resistance won the first round that wasn't a one-sided slaughter of weaponless humans. The aliens in thier Dreaming see a feild of crystalline gossamer asteroids and a thin, crescent shaped ruin of what once was a lunar base, and for the first time, they are starting to show a recognizable human emotion: Fear.
They have adapted, and shifted form, to reflect the new realities of the war: they are now things that go bump in the night, things you see out of the corner of your eye, then, in a flash, a friend has been reduced to scattered blood and entrails.
You might catch a glimpse or feel the touch of slimy tentacles with which they bind or violate. You may see the flash of jagged claws with which to catch and rend. You may hear sonar screeches or eerie lupine howls.
You will almost never see the whole creature unless it is cornered, and then they will be traditionally monstrous, with distended proportions, glossy translucent skin and glowing red eyes behind emotionless white masks.
Think of the Horrors of Dunwich. Think of the Angels and the Mulians. Think screaming hungry ghosts that can only be laid to rest with zanpakuto or a Quincy's glove. Then add a heavy dose of the Big Bad Wolf. That is what stalks the dreams of the new generation.