Thick, rust colored sludge
One of the inspectors here said it reminds him of the sludge from Ghostbusters 2 that flows through the sewers. Its very thick, sometimes all you'll see is the stain. When it dries up, it can look very similar to clay. This is why most people think that it is just a clay deposit in the water.
Living organism
The biggest problem you will face if you see iron ocre (other than it clogging your sump pump) is that it is alive. If you have been reading along in the DryZone pics and blogs, you will know that anything that was once alive can grow mold. This means that iron ocre has be be controlled and cleaned up, otherwise its a mold factory.
High humidity in the crawlspace
Mold needs 3 things to grow. The mold spores are always in the air, nothing you can do about that. The food source, which is anything that was once alive (wood, paper, plant life). Its hard to build a house without organic things like wood. The last component, and the easiest to control, is the moisture or humidity. Mold needs to "drink" just like other plant life. It gets water through the humidity in the air, therefore if you lower the humidity in the crawlspace, you will lower the chance of a mold outbreak.
River of filth
What you are seeing is about 2 feet wide and completely surrounds the crawlspace. It is basically a river of sludge. Sort of reminds me of a stagnant pond. I know I wouldn't want this under my house, would you?
Clogged sump pump discharge line
This is a cross section of a pipe clogged by iron ocre. Looks a lot like clay, doesn't it? There are great treatments to prevent this from becoming an ongoing problem. The first step is to get a fresh sump pump and discharge pipe. There is just no use trying to save this pipe.