Student accounts are grayed out on the Manage Students page if a task is enqueued to be performed on that student's account. Tasks could include merging the student's account with another account, changing the student's class, or deleting the student from a class. If the student just became grayed out, try refreshing the page. If the student remains grayed out for several minutes, please reach out to firstname.lastname@example.org.
The student will not be able to submit assignments while they are grayed out. Because student merges occasionally get stuck in a long queue, we recommend waiting until the end of day to merge duplicate student accounts, so as to prevent blocking students from being able to submit.