Preheat the oven to 350 degrees F. Bring a large pot of salted water to a boil. Add the pasta and cook as the label directs, adding the cactus during the last 1 minute of cooking. Reserve 1 cup cooking water, then drain. Return the pasta and cactus to the pot.
Meanwhile, spread the pepitas on a baking sheet and toast in the oven until golden, about 8 minutes. Let cool 5 minutes, then transfer to a blender. Add the cilantro, garlic, olive oil and 1/2 cup of the reserved cooking water and puree until smooth.
Add the pepita pesto, tomato, chile and all but 2 tablespoons of the cheese to the pot with the pasta and cactus; toss to combine, slowly adding the remaining 1/2 cup cooking water as needed to loosen. Season with salt and pepper. Serve topped with the remaining 2 tablespoons cheese.