Dry the salmon well with paper towels and season it all over with salt. In a small bowl, combine the chili powder, garlic, olive oil, and lime juice; stir to form a paste. Spread the paste all over the salmon fillets in a thin layer, distributing it evenly. Set aside at room temperature for 15 minutes. Meanwhile, make the sauce. Stir together the cilantro, green onions, crema, lime zest and juice, and a pinch of salt. Taste and adjust the seasoning with more lime juice or salt as needed. Transfer to a small serving bowl and set aside. Set a large nonstick or well-seasoned grill pan over medium-high heat and wait for it to get hot, about 5 minutes. Brush the surface with vegetable oil and add the salmon fillets skin-side down. Reduce the heat to medium-low and cook, pressing down on the fillets with a spatula to make sure the skin is making contact with the pan. Cook until the skin is crisp, about 6 minutes. Turn the salmon and cook on the flesh side until the salmon is done to your liking (about 1 minute longer for medium-rare). Transfer the fish to a warmed serving platter and cover loosely with aluminum foil. Let it rest for 5 minutes before serving with the sauce.