Good morning, @SeanKim ...
I believe you will need to go to the SpeedGrader for this particular quiz and go through each student that has the icon that you are seeing in on your "Grades" page. When you go to the SpeedGrader for the quiz, you may see some messages at the top of the screen (in a reddish-pink color) saying that some questions need to be graded (for example, essay questions). For example:

Make sure that each question in the quiz has a score, and then at the very bottom of the questions, there should be an "Update Scores" button. Go back to your "Grades" page (refresh your screen if necessary), and this *should* display a score there instead of that icon. Go through each of your students, and then you should see a score there on your "Grades" page.
I hope this helps a bit. Sing out if you have any questions...thanks!