The green online icon indicates that a captured session has had activity in the past 30 minutes. We will add online sessions to the Everyone segment as soon as we know they exist, although the events may not yet be indexed. This means those online sessions might not show up in your search results if the events you're searching have not yet been indexed.
The blue icon with the date indicates that the session is complete (we close sessions after 30 minutes of inactivity - more information about how we define a user session here) and has been indexed. All events in these sessions will return in your search results.