Preheat the grill to medium-high heat. Grease the grates with oil.
Place the sweet potato slices on the hot grill. Grill for 8-10 minutes, flipping once halfway through, until lightly charred and tender.
In a small bowl, whisk together olive oil, garlic powder, smoked paprika, cayenne pepper, salt and pepper. Brush the sweet potatoes with the spiced oil.
Remove from the grill and transfer to a serving platter. Sprinkle with parsley and feta cheese. Serve warm or at room temperature.