Q: Green light heart beat indicator

Hi

If the green LEDs are permanently lit, you may have a workout or other fitness-based session in progress (there may also be a runner icon on your watch face).

In the Workout app - and also any third-party fitness tracking app that you may have installed - be sure to end any workout that is currently paused or in progress:

• Work out with your Apple Watch - Apple Support
• https://help.apple.com/watch/#/apd36e57e00c

If the icon remains / the LEDs remain lit up after ending any active workout, restart your watch:

• Restart your Apple Watch - Apple Support

More information:

Status icons on Apple Watch - Apple Support

Note that it is normal for the heart rate sensor to take background measurements of your heart rate regularly throughout the day, but the LEDs do not remain permanently lit for this purpose. 

More information:

Your heart rate. What it means, and where on Apple Watch you’ll find it. - Apple Support

You're welcome!