Hi do you have a "Grade to pass" shown under "Grade" for this quiz, or maybe did you when the student sat the quiz?
These settings are little fiddly but what seems to happen is that, if there is a grade present in "Grade to pass", if the student takes the Quiz and does not get that grade, the Quiz will then show a red "X" for "complete but fail". If they do get the grade, it shows a green check mark for "Complete and pass".
If the student hovers over the "fail" icon they see some explanation
The help icon below suggests that this is the way it's intended, where it says "Pass and fail icons may be displayed if a pass grade for the activity has been set".
Also I get the same behaviour if I try this in a "Topics" course, so I think it's correct, though it's possible I am missing something.
I also notice that, if you remove the "Grade to pass" after the student has sat, the red Xs still appear - you have to clear their attempts and have them sit again (or maybe you could clear and then manually re-enter their grades, but I didn't try that)