By default, the M4 eyes code only runs the eye animation. You'll need to turn on a setting in the code in order to enable NeoPixel animations alongside the eye animation.
In the M4_eyes Arduino sketch, look in the tabs for the user_neopixel.cpp sketch. In the top of the code, change the 0 to a 1. This will enable the NeoPixel rainbow cycle code to run while the eye is animating.
Update the LED_PIN to 3 and change the LED_COUNT to 30 or however many you'd like to use in your build.
Adjust any of the parameters you see fit. For example, the brightness of the NeoPixels (default is 50, 255 is the max).
Re-upload the code to the HalloWing M4 using the recommended profile and configuration.