Preheat a grill to medium-high. Slice each block of haloumi lengthwise into thirds, creating 6 slabs. Brush the watermelon with EVOO. Grill both the haloumi and watermelon, turning occasionally, until grill marks appear, about 3 minutes per side. Cut the watermelon from the rind, then cut into 1-inch pieces.
Divide the haloumi and watermelon among plates, drizzle with the lemon juice and sprinkle with mint. Serve immediately.