Essay questions need to be manually graded within Exam Reports. When viewing the exam, a Grade Exams button will be listed within the Completed section if there are Essay questions that need to be graded. Clicking this button will direct you to the Ungraded tab within Exam Reports
By default it will prompt you to grade by Grading Students. When grading by student a list of student attempts will be listed.
If an exam contains more than one essay question, it may be beneficial to use the Grading Questions button to grade by question rather than by student.
This will allow you to grade all submissions to one essay question one after the other by using the Next and Previous buttons to move between essay question submissions.