Hi! To do that, you can open the PNG file in photoshop (or any other photo editing software), and remove all the colors but the green. It might be tricky to find where it is in the texture because it's a very tiny band of colors, but by zooming in you'll be able to find it. Then, once you've blackened all the colors but the green, export that and use it as the emissive map in Unity and you're good to go!