This is part of the hardware used to measure heart rate. The light flashes on the blood pulsing under your skin and a light sensitive photodiode measures the amount of green light absorbed. The amount of light absorbed at a given time determines your heart rate.

You may notice the green LED light flashing at random times or while you're not wearing the device. This is normal. After roughly 15-20 minutes of non-activity (little to no movement), the device will take an automatic resting heart rate reading. These automatic readings are averaged, and displayed as a single data point within the heart rate chart in the Striiv app. 

For more info on how to use the heart rate monitor, reference the following article: