The participants page in Moodle includes both active students and students who were at one time in the course, but have since dropped the class. These students who are no longer in the course are marked with an orange "Suspended" badge. Students who are "suspended" can no longer access the course and do not show up in the gradebook.
"Suspended" students are kept there for data loss prevention reasons; if a student were to drop a course their data (assignments turned in, grades received) would be deleted if we removed them from the course entirely. This could be problematic if they were to re-add the course later.
How do I get a list of just the "active" students in my class?
The easiest way to get a list of current students in your course is to look at the gradebook in Moodle. You can view the gradebook by clicking on Grades in the left sidebar of any Moodle course.
If you need to use the participants page (ex: you are projecting your screen in class and you need to pull up a list of students) and you need to hide the "suspended" users, you can do so by clicking the dropdown menu on the participants page, and choosing Status: Active. This will only show students who are still active in the class. Unfortunately this setting is temporary and does not stay that way when you revisit the Participants page later.