Preheat grill to medium-high. Whisk lime juice, 1 1/2 Tbsp. oil and cayenne in a small bowl. Season with salt.
Brush corn with remaining 1 1/2 Tbsp. oil and grill, turning a few times, until kernels give slightly when pressed and turn deeper yellow, about 12 minutes.
Remove corn from grill to a serving platter and brush with lime juice mixture. Crumble queso fresco on top and sprinkle with cilantro, if desired.
