While Moodle allows you to hide items in the grade book, we don't recommend doing so and here's why:
Hiding Graded Items
If you want to hide existing assignments or quizzes from students because you don't plan to incorporate them in your course this term, we recommend that you remove the points attached to them and save your changes. Once the items have no points attached, they should no longer appear in your grade book.
Avoiding the "Hide" Option
When you open the Edit pull-down menu, you'll see a Hide icon, designed to make the item (and any associated grades) visible to you, but hidden from students in their grade reports. We don't recommend using this feature because the item is not all that gets hidden—when you hide an item in the grade book, students will no longer see the course or category totals when they check their grades. This can lead to confusion for students, and reduces the usefulness of the grade book as a tool for students to track their progress in the course.
It's important to know as well that if you hide a gradable assignment, quiz or forum (that is, one with points assigned to it) from students on the course main page, the item will also be hidden in the grade book, resulting in hidden totals and likely student confusion.
Bonus: What Do Students See?
If you're curious about what students see when they check their grades, click on the User report link under the tabs at the top of the page, then select a student's name from the pull-down menu in the upper right corner. You'll see a close approximation of what students see (naturally, students see only the grades assigned to them).