Yeah, 18 weeks seems way beyond what I would consider a case of pasty butt. Can you or a helper lift the feathers to expose the vent and take anither picture so we can get a clearer idea of what's going on backk there?
Also, is this a laying bird? Do you know when she last laid an egg and if it was normal? How's her behavior, is she eating, drinking, pooping normally? Does she appear depressed or in pain? Is her tail up or down? What are you feeding, including treats? Can we get a pucture of her poop?
Feel her crop at night and again in morning before she has access to food or water. Describe how it feels in the morning. Full or flat (empty)? Doughy, spongy, soft, like a water balloon, lumpy, etc?