The green LED for one bank has failed, most likely. The red LED turns off when it should, but the green one isn't up to the challenge. Check this by swapping two sets of +/- wires connected to the "indicator panel". Swap connection locations for one "good" battery, and the "bad" (no green) battery.
If the blank LED location remains in the same place after the connection wire swap, the indicator panel is faulty.
If the blank LED location moves after the wire swap, the "bad" battery is in fact faulty; it isn't reaching a voltage high enough to trigger the panel to turn on the green LED.
Are the batteries the type with removable caps? (generic add-water type battery?) If so, the absolute most accurate way to know the condition of a battery is to check the specific gravity of each cell of the battery. A specific gravity test gauge looks like a glorified turkey baster and often costs not much more than $10. (Don't get the small syringe style hydrometer with floating ball, as those are not so good.)