I explored around Molino Canyon Overlook today, and I discovered a number of repulsive, yet almost beautiful puddles completely filled with viscous, jelly-like, brown algae.
A small amount of nice, clear water is now flowing in the stream bed here, but stagnant puddles left over from the rains remain, and many of these puddles have been completely overrun with slimy, brown algae. The surface of these algae pools looks almost metallic and is covered with a rainbow sheen like that of oil on water.
It's possible that the rainbow sheen is from oily decomposition products released from dying algae. I don't know how toxic this stuff is, but I kept my dog well away from it.