If you marked your course as "private" it will not show in Google or any other search engines. Private courses are only visible to those whom you share the URL of the quiz with.
If the course was first public and then marked as private, it's possible that Google indexed it earlier. We add the noindex and nofollow directive for Google (and other search engines) on private quizzes so next time it refreshes, it should remove the quiz from search (a process that can take a few days or can also be done instantly - depends on Google).
If you want it to be removed immediately you can:
Related Articles:
How do I make my courses private?
How is course data and user information protected?
How to set password and accessibility for my courses