Why does my Apple Watch glow green sometimes?
It's taking your heart rate.
Per Apple:

&quot;Blood is red because it reflects red light and absorbs green light. Apple Watch uses green LED lights paired with light‑sensitive photodiodes to detect the amount of blood flowing through your wrist at any given moment. When your heart beats, the blood flow in your wrist - and the green light absorption - is greater. Between beats, it's less. By flashing its LED lights hundreds of times per second, Apple Watch can calculate the number of times the heart beats each minute - your heart rate.&quot;


The green LED's actually are flashing hundreds of times each second so to you it looks like it's glowing, but it's not.