Why is the green light at top left turn on?

As I was using my DT2 phone while on the charger, the green light came on and then the phone shut down and the light stayed on. Turned it back on and looked at the settings for green light info...nothing found. When I started using it while on charger, the battery had 54% power. Upon turning phone back on with the green light on, 21% power. How do I turn this light off?

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