The Glass Fairy is the magical equivalent of a leech, feeding on the extremely small bits of solid magic that exist in the blood of magical creatures. They have a spell in their bodies that makes it so the victim can’t feel them walking on them and another which muffles the sound of their flight and movements. Combined with their clear bodies they can be very hard to detect. They use a spring loaded needle-like appendage in their forelimbs to pierce the skin and suck up blood.
Since magic blood doesn’t necessarily have to be red, it was actually a long time before humans knew they ate blood. From the point of view of those they didn’t feed on, glass fairies just looked like they were periodically filled with colorful liquid.