This disgusting-looking stuff is sewage fungus, and grows when nutrients contaminate the water. In this case, judging by the state of fungal growth, the pollution's chronic; in other words it's been going on for months.
This part of the brook - actually a tributary into the main water course - doesn't have a lot in the way of an "upstream", so one of the places the Environment Agency are looking at is the new anaerobic digestion plant owned by Grocontinental which is in the next field. Its possible that the maize stored on that site has been allowed to leak nutrients into the ground and they've ended up in the water course, causing the problem.
Worryingly, the brook runs right through town and into Greenfields Nature Reserve at Chemistry, with water vole colonies all along the length. A lot of people will be watching for the outcome of this.