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.
While I wait for the water voles, it's nice to check out the birding action. These photos are from Ifton Meadow reserve, near St Martins: skylarks and linnets and meadow pipits showing well, and plenty of goldfinches and possibly a pair of pied flycatchers in the trees. The fungus is Elf Cup.
This blog charts the fortunes of water voles in and around the Whitchurch area, North Shropshire. Water voles are one of the UK's most threatened mammals, extinct in many counties, and so it's vital they receive as much monitoring and protection as there is going. Here in Whitchurch we're lucky enough to have them right in the middle of town - how cool is that?