Whisk together first 6 ingredients in a small bowl. Combine broccoli slaw, squash, and bell pepper in a large bowl. Add half of mayonnaise mixture (about 1/4 cup), tossing to coat. Cover and chill both slaw mixture and remaining mayonnaise mixture at least 2 hours or up to 24 hours.
Drain slaw mixture, just before serving, discarding excess liquid; return to bowl. Add reserved half of mayonnaise mixture and pecans, tossing to coat.