From the Fountain Paint Pot trail.

From an NPS website:
Bacteria and other thermophiles(heat loving microorganisms) usually form the ribbons of color like you see here. The green, brown, and orange mats are cyanobacteria, which can live in waters as hot as 167 F (73 C). At this temperature they are usually yellow-green. They become orange, rust, or brown as the water cools. In cooler water other thermophiles may appear that will modify the colors even more. Color may also change due to stress, such as the intense sunlight of mid summer.