The white circle indicator for active sessions, should really have a 1-2 pixel border of another color. On sessions with auto-screenshot enabled, where there is a lot of white already in the thumbnail image, the indicator is completely invisible.
I would personally recommend a calm green (for "good" or "on" or whatever), a nearly-black shade of gray, or possibly if you want to get creative... a dynamically chosen color based on a complementary color of the "average temperature" of the pixels behind and around the indicator.