Green lights on back of watch
How to turn off green lights on back of watch
Apple Watch
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Those are al the Apple Watch sensors and allow the watch to work the way it does to detect heart rate and other body functions it reports.
If you really want to turn them off, go to Settings->Privacy->Health, and turn off Heart Rate.
If they are going off while the watch is not being worn, then make sure there is no workout currently active on the watch as that can cause the watch to attempt to get the heart rate and turn on the lights.