#import<FastLED.h>

#define NUM_COLS 20
#define NUM_ROWS 20
#define NUM_LEDS 256

CRGB leds [256];

void setup()
{
  LEDS.addLeds<WS2812, 3, GRB>(leds, 256);
}

byte idx = 0;
void loop() {
  leds[idx] = CRGB::Red;
  FastLED.show();
  leds[idx] = 0;
  idx++;
  delay(10);
}