It's a piece of forbidden contraband the surfacedwellers refer to as a SUGAR CUBE.
A gift from TC, who's always trying to get you to engage with new, mirth inducing, highly illegal things.
However, you neglected to tell him that concentrated sugars are HIGHLY TOXIC to nymph anatomy. One bite of this and you'd be in big trouble. Any sane nymph would cast these AGENTS OF THE SWEET onto a dumping beach and forget they ever existed.
But they're from a friend, so the risk of temptation is something you're willing to endure.