Curry powder is a spice blend. Most contain turmeric, coriander, cumin, fenugreek, and chili peppers of some sort. From there, they vary depending on the region of origin and, of course, the recipe. Other ingredients can be garlic, ginger, fennel seed, cinnamon, cloves, mustard seeds, cardamom, nutmeg, to name a few possibilities. I generally make my blends. It's a great way to put spices in to use, and it gives me control over the ultimate flavor. A little research into typical spice blends by region helps guide what I use.
This recipe also includes kaffir lime leaves. They are a typical ingredient in a green curry paste and those paired with the lemongrass give the broth a citrus background. Together they keep the soup from seeming "heavy" which could happen with the many spices in the curry powder.
I should note that turmeric's health benefits are enormous, so including the spice in your diet regularly is a great idea.
This recipe is quick to put together, which makes it a lovely candidate for a weeknight meal. I serve it over glass noodles, either rice or bean thread, but consider udon or plain rice.