In American university systems of my acquaintance, we always call them “graders”. Remember though that in America, the action of assigning a grade to a student’s work is called grading. One does not normally speak of assigning a mark, just a grade. So “marker” wouldn’t be used this side of the atlantic.
I’ve at times been a grader myself, even though I was not the professor nor the TA (teacher’s assistant) for that particular course. This tended to happen towards the end of the term when there was more material needing grading than they were staffed up to handle. This was for undergraduate courses in computer programming.
Also, even though we always assigned numeric scores between 0–100, we still called them graders, never *scorers. Exactly how a particular numeric score mapped to a letter grade depended on the university, the course, and sometimes also on the thing being graded.