The flashing green LED means that the unit has gone into an alarm. This is referred to as the alarm memory condition, and it will continue to flash until the reset button has been pressed.
The green LED indicates your alarm has power. The red LED will flash around once every minute indicating you have battery backup power. If the alarm sounds, the red LED on the device that initiated the alarm will flash rapidly. The red LED on all other interconnected alarms will remain off.
After the alarm, the unit will Latch, or recall which unit initiated the alarm. The green LED will be on for two seconds and off for two seconds. You can reset the latching feature by pressing the test or silence button.
Check out the smoke alarm safety tips at National Fire Protection Association.