First of all, you didn't provide any info concerning what library you're using and where you got stuck.
So I'll just assume you're using FastLED, and since the issue isn't super complex, I can give you pretty much all the code you may need.

Looking at the drawing, the LEDs are positioned like it is one single strip.

The main line is 14 LEDs (LEDs 0-13), and at LED 10, we see 2x 5 LEDs (14-18 and 19-23), making it a total of 24 LEDs (the LED array starts counting at zero, so 0-23).

The base pattern looks like this, when initially skipping the fact that the LEDs fade out:

0
1
2
3
4
5
6
7
8
9 + 14 + 19
10 + 15 + 20
11 + 16 + 21
12 + 17 + 22
13 + 18 + 23

We can do this in code (fading/clearing of LEDs not yet included):

  for(int i=0; i<14; i++) {
    leds[i] = CRGB(0,255,0);
  
    if(i>8) {
      leds[i+5] = CRGB(0,255,0);
      leds[i+10]= CRGB(0,255,0);
    }
  
    FastLED.show();
    delay(speeddelay); // determines speed, higher number = slower effect
  }

So for LEDs 0 to 13, we set the LED to green.
If we reached LED number 10 (remember, we start counting at zero and not one, so LED 9, eg. >8, and up) we need to go sideways as well (14 to 18 and 19 to 23).

Next we need to add a fade to the code (you may need to experiment a little with the value "96" to get the desired amount of fade-out effect):

  for(int i=0; i<14; i++) {
    fadeToBlackBy(leds, 24, 96); // led array, number of LEDs, amount of darkening

    leds[i] = CRGB(0,255,0);
  
    if(i>8) {
      leds[i+5] = CRGB(0,255,0);
      leds[i+10]= CRGB(0,255,0);
    }
  
    FastLED.show();
    delay(100); // determines speed, higher number = slower effect
  }

So,... all combined this should be something like this (untested, as I do not have my hardware handy at the moment), with the delay and color as a parameter:

#define FASTLED_INTERNAL    // just used to mute the Pragma messages when compiling
#include "FastLED.h"
#define NUM_LEDS 24
CRGB leds[NUM_LEDS];
#define PIN 6

void setup()
{
  FastLED.addLeds<WS2811, PIN, GRB>(leds, NUM_LEDS).setCorrection( TypicalLEDStrip );
}

void loop()
{
  cross_effect(100,96,CRGB(0,255,0));
}

void cross_effect(int speeddelay, int DarkfadeAmount, CRGB LEDColor) {

  for(int i=0; i<14; i++) {
    fadeToBlackBy(leds, NUM_LEDS, DarkfadeAmount); // fade all to black one step at a time
       // values: led array, number of LEDs, amount of darkening

    leds[i] = LEDColor;
  
    if(i>8) {
      leds[i+5] = LEDColor;
      leds[i+10]= LEDColor;
    }
  
    FastLED.show();
    delay(speeddelay); // determines speed, higher number = slower effect
  }
}

Again: untested, and you may need to tinker a little with the values.

Thank you very much, it works great!

I was using neopixel, but now I also added fastled, because I didnt know similar method to "fadeToBlackBy". Is there maybe something similar for Adafruit_Neopixel?

Previously I just set the leds off like this: 

uint32_t off = strip.Color(0, 0, 0);

if (i >= loopLED) {
       strip.setPixelColor((i - loopLED), off);
       color = strip.Color(0, i - 5, 0);
   }

You're right: NeoPixel doesn't have a function like that.
This is why I recommend most to use the FastLED library. It is more mature, faster and has more functions.

You could try to write your own code to mimic the FadeToBlackBy function for NeoPixel, but personally I would stick to FastLED 😊 

In your code you set the LEDs to black. Is that what you were looking for? (since the animation suggested you'd want the LEDs to fade away instead of instantly going off)

@hans because I like this fade effect, I will switch my whole program to FastLED, I think it will be more sustainable, than programming fade function by my own. (especially because I am not so good (yet) at it 🤣 )

Sounds like a good plan. 👍 😉 
I'll admit; I once started with NeoPixel as well until I noticed how much faster and better FastLED seems to be (especially noticeable with very long LED strips).

I hope the code works for you!