Latest dump1090-mutability I assume. These are aircraft where you have previously received a position (mlat or ads-b), and you are still hearing messages from the aircraft, but there haven’t been any positions available for >60 seconds. After 60 secs without a position they will turn white and the marker will be removed from the map. You can get the historical track by selecting the row in the table.
This usually happens because the aircraft is no longer in range of 4+ receivers for mlat - often happens as they descend. It can also happen with ads-b aircraft on the very edge of your range, where you can decode the shorter altitude/squawk replies but not the longer position messages.