After you rule out or correct conditions that cause loose poop, sometimes butts can become crusty normally due to eating certain types of food such as squash.
Every week or so, I set up a couple of small wash tubs with warm water and children's bubble bath soak. I get a firm hold of the candidate who is in need of a tune-up with the head under my arm. I back her up to the wash basin, with her butt hanging over the edge. With my free hand I splash the sudsy water onto the crusty area until it softens. Then I use my fingers or a wash cloth to pull off the accumulation.
The best thing about using this children's bubble bath soak, no-tear's formula, is it requires no rinsing. I use a towel to dry the area, or if it's a cold day, I blow dry the butt. They really enjoy that! Even the roosters! It's not necessary to immerse the entire chicken, either.
This method works well for dirty feathered feet, too.