On Arduino using the M5Stack (in my case the Gray version) pin 21 is the pin to pass on to FastLed. For M5StickC it is pin 32.
That the M5Stack shows a bright white light for the first LED seems to occur when one also uses the M5.Power.begin() command.
This piece of code should work on both devices adjusted for the device in use:
FastLED.addLeds<NEOPIXEL, 21>(leds, 15); // M5Stack Gray
//FastLED.addLeds<NEOPIXEL, 32>(leds, 15); // M5StickC
static uint8_t hue = 0;
if (255 == hue)
hue = 0;
FastLED.showColor(CHSV(hue++, 255, 50));