That looks abnormal and is likely to be some form of pollution. Surely the scientists at the Environment agency will be able to identify what substance is in the water which shouldn't be there and do something about it.
I remember the London reservoirs used to turn that colour when they did something to kill off weed or algae. It didnt kill fish. The roach went crazy on the feed when it subsided.
They used to call it something like "stone walling" or similar. Someone will remember.
I thought it might be Methylene Blue, but looks more like paint than anything.
We had white paint dumped in our brook by some bright spark, it soon cleared and didn’t seem to do any lasting damage. EA took samples but said they would be more worried if it was milk