what is snow sugar? my icing sugar dusting keeps vanishing

Very real product, and it exists precisely because of the heartbreak you just described.

Regular icing sugar is fine sucrose - it dissolves on contact with ANY surface moisture. Cream, fruit, a moist crumb, monsoon air, doesn't matter. Snow sugar (also sold as snow powder or neige decor) is dextrose blended with starch and a little vegetable fat. That fat coating repels surface moisture, so the dusting stays bright white for a day or more, even on cream and cut fruit. Bakery ingredient shops and the big online baking stores all carry it, it's not expensive.

If you're stuck with regular icing sugar: dust at the absolute last moment before handover, driest surface possible, and accept it's a two-hour finish. On anything properly moist the physics just wins. Desiccated coconut or a stencilled cocoa dusting are the old-school keeping-white workarounds.