A description of the operation of the LEDs on the Mayfly board can be found here: https://www.envirodiy.org/mayfly/hardware/details-and-specs/
For the one you asked about, it’s under part C:
C- Power LEDs and DIP switches: LEDs indicate board power (green) and USB power (orange). Useful for easily seeing if the board is on and if USB power is connected. Green LED will be lit anytime the board has power and the power switch is in the ON position. Orange LED will be lit anytime power is being supplied through the USB or FTDI connectors. If Mayfly board is deployed in a sleeping logger station, it is recommended to set both DIP switches to the OFF position in order to save battery power. Use a small pointed object to gently slide the small white squares of the DIP switch either to the ON or OFF positions.
Have you tried replacing the battery back on the station with a fully charged one? Sometimes the Lipo packs won’t take a charge or charge properly and just need to be replaced with a new one.