I would like to offer a solution to improve Moodle icon usability for activities.
The problem is that default activities icons cannot reflect different purposes. Even if you set custom ones to /pix directory it is a limited solution.
So, it is better for usability and user experience to have an option to set a different icons that in the teacher`s opinion suit best to an activity resource they add.
For example, for an educational quiz you might prefer an icon , and for the certification final quiz .
It is obvious that icon flexibility will add necessary usability both for teachers and students.
Once you create any activity module there should be an option to choose an icon from (https://fontawesome.com/):
smth like this: