The brown substance in the water is distinctive once you've seen it a few times. Here's another view, a few seconds later (below). Note how it changes shape, and that it doesn't dissolve or disappear right away:
And the answer is? Well, here's the animal responsible for it:
The brown substance in the water is octopus ink. This octopus released a cloud of ink while jetting across a shallow tidepool. Afterwards, the octopus settled on the bottom briefly and then crawled under a rock.
Octopus ink is a combination of pigment, e.g., melanin (released by the ink sac) and mucous (released by the funnel organ). It's quite effective as a distraction. The ink morphs into a variety of shapes, and stays visible for several minutes — giving the octopus plenty of time to escape while a potential predator is confused by the ink.
You might have seen some cephalopod ink that is black. Ink color varies depending on the species. Whenever we've seen Octopus rubescens ink, it's been orange-brown in color.