Why does my apple watch have a green light on my apple watch when I rub my eyes.

Are you referring to the back of the watch? 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. For more info, see:

Monitor your heart rate with Apple Watch - Apple Support