Moonmilk deposits, Wind Cave National Park, 2015.

Moonmilk is a white deposit of aggregates of very fine crystals. It's a common cave deposit that probably precipitates from dripwater entering the cave. It's gooey when wet (like cream cheese) and crumbly and powdery when dry. As expected, moonmilk is made of carbonate materials like calcite, gypsum, hydromagnesite, etc. Microorganisms like bacteria, algae and fungus have been found in some moonmilk deposits (but not all) and may play a role in its formation.