What icing has a thin crust and moist interior?

The icing you're referring to is likely a "glaze." A glaze typically has a thin, shiny crust that forms on the surface as it sets, while the interior remains moist. It is often used on pastries and cakes to add flavor and a glossy finish without overwhelming the dessert's texture. Glazes can be made from various ingredients, including powdered sugar and liquids like milk or citrus juice.