You need a loop to turn all the pixels on one color (red).
Then, you need a loop that moves a white LED through 24 positions, taking the required time. As that loop turns a pixel from white to another color, you need to decide whether the other color is red or green.
You need a variable that keeps track of where the green LEDs start. Each time the cycle completes, you increment that value. If the white position is moving from an pixel below that number, set the pixel back to green. If the white position is moving from a pixel above that value, turn that pixel back to green.
What is to happen when all the red pixels have been turned green? Does the white pixel keep rotating?