Delicate structures made of ice crystals that form on top of frozen bodies of water are usually referred to as frost flowers. If the frost forms on solid objects or on window glass it is usually referred to as hoar frost and window frost respectively. Casually they all can be called frost flowers.
Frost flowers at Milliken Park formed on top of the ice on a puddle after weeks of really cold (below -20C) temperatures.
When the atmosphere is much colder than the ice temperature, supersaturated vapor near the ice surface freezes (water molecules deposit directly from vapor as solid ice) and forms delicate feather-like crystal structures which looks very different from large chunks or sheets of ice formed when liquid water freezes.
Note: To make things somewhat confusing the English term “frost flower” can refer to an unrelated phenomenon where ice is extruded from plants when they are getting frozen.
Further Readings:
Frost on Wikipedia.
Ice crystals on Wikipedia.
News – Rare ‘frost flowers’ form on B.C. lake – The Weather Network
Hoar Frost Pics – And WHY Is It Called That?! : KTTC Weather Blog
Photos: Hoar Frost Vs. Rime