Neither microbes nor algae. It's Antimony and Arsenic sulfide. That pool is 75C and pH 5.5. The microbes are actually present at the very edges of the pool at water level. The white outcroppings all around are silica, and they look the way they do because microbes colonize the edge of the pool at water level. The pool has lots of silica in it which crystallizes on the microbes and eventually covers them completely (winds affect the water level) and then they are silicated on the inside too. The little white stalactites are actually multiple layers of entombed microbes.
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