Green flash is spoke magnet passing the sensor, red flash is cadence magnet passing the sensor. I'd guess the sensor and magnets are setup correctly and you need to pair it on the device under the correct ANT+ profile. I believe that's "Speed/Cadence" for GSC-10 but read this for more details.

The ANT+ Bike Speed/Cadence Sensor: Everything you ever wanted to know | DC Rainmaker