If you have an OG (first gen) TRMNL, an LED on the back lights up when the device is plugged in + charging. This visual cue is by design, and makes it easy to unplug when charging is complete.
However, some users prefer to keep their device always plugged in. For you, we have a suggestion to prevent green LED bleed, or damage to the screen (pixel bleed).
First, acquire a reflective or thick material such as aluminum tape:
Next, safely open your TRMNL enclosure and apply this material to the ~1x1 inch area where the LED sits behind the screen. You can plug in your device to compel the LED to turn on and identify a proper location.