In the D2L Grades area you might notice two types of conversation bubbles, or Discussion-related icons:
The first set of conversation bubbles (depicted in dark gray) indicate that there are discussion posts from the student that have already been marked as read in the Discussions tool area.
The next set of green and orange conversation bubbles indicate that the student has posted a new (and unread post(s)) in that discussion.
Clicking on the icons will allow you to navigate to the Discussion post(s) for that student.