In a small, dry skillet over medium, toast the chili powder, stirring, until fragrant, 1 to 2 minutes. Transfer to a large bowl and let cool completely. Whisk in the the lime zest and juice, agave and ¾ teaspoon of salt. Add the watermelon, cantaloupe, cucumber and mint; toss to coat. Cover and refrigerate for at least 15 minutes or up to 30 minutes. Meanwhile, in a small bowl, toss the queso fresco with ½ teaspoon salt and the pepper. Using a slotted spoon, transfer the salad to a serving bowl and top with the queso fresco.