Yes, it's a thing. It's what happens when you grill watermelon; it makes it look likes it's been tattooed. But it's delicious. You can't see the tattoos on ours in the picture, but the faint lines are there!
Slice a watermelon into thick slices. Brush with some olive oil (but not too much! You don't want it greasy!), and sprinkle with salt and pepper. Grill over medium high heat about 2 minutes per side. Watch it carefully. If it gets too hot, the liquid will gush out, and nobody wants that! Serve. Enjoy!