Spray an outdoor grill with non-stick cooking spray and set the heat to medium-high. Place the tilapia on the grill and grill for about 4 minutes per side. Remove from the grill once fish is done (it will flake with a fork).
Mix together the chopped cilantro, lime juice, lime zest, and salt in a medium bowl.
Smash the softened butter into the cilantro lime mixture and mix it until the cilantro is well distributed.