It’s just water. Cold water on a warm hand, to be exact.
This is an image from an infrared camera I was testing today. It turns out that humans glow in infrared, exactly in the same way that a red-hot piece of iron glows in visible light. We’re just a lot cooler than the red-hot iron, so we glow in much lower-energy infrared light, compared to higher-energy visible light. So this camera, sensitive to infrared, shows anything that’s warmer than room temperature as a bright spot. The water on my hands was cold, so it looks dark by comparison. At the left is strands of my hair (cool and dark) against my shirt (slightly warmer).
So yes, the camera works. Now I will use its powers for science!