When you delete a clip from the media pool, normally it leaves the corresponding entry in the master session so all the grades with that clip don’t get lost. If you have any EDL events conforming to this clip, it will show as Blue Crosses. It tells you that you had a clip earlier which conformed to this event, but it’s been removed from the media pool.
Now if you add a newer version of that clip with matching timecode extents, it will replace the orphaned clip in the master session, and you will recover all the grades and timeline information automatically. This is quite useful for replacing individual shots with newer versions of the shots from VFX. Of course, you can add both versions, and use the Conflict Resolution window to resolve the conflicts.
If you want to delete the clip, and also delete the corresponding entry in the master session, there is an option to delete from master session as well.
A Red Cross means there is no matching clip in the media pool and/or the master session.
Regards,
Rohit