For the salad: Whisk together the lemon juice, olive oil, agave, Dijon and 1/4 teaspoon salt in a bowl.
In a large bowl, toss the arugula, cilantro, tomatoes, onions and 1/8 teaspoon salt. Before serving, drizzle the salad with vinaigrette and toss.
For the snapper: With a sharp knife, make two shallow cuts, about 1 1/2 inches apart, on the skin side of each fillet. Season the fillets on both sides with the crushed red pepper and 1/2 teaspoon salt.
Place a large skillet over medium-high heat and coat with the grapeseed oil. Place the fish, skin-side down, in the skillet and cook until golden brown, about 4 minutes per side. Transfer to a paper towel-lined plate.
Serve the snapper fillets with the salad.