This sounds very like The Clone by Theodore L. Thomas and Kate Wilhelm, but in that book I don't think the slime was red and it was published in 1965 so that's 15-20 years earlier than your timescale.
I last read the book in the 70s, and it's so obscure that Googling hasn't found much to jog my memories. However the slime did start in the sewers/water supply but I think it was accidental - an unhappy mingling of chemicals that started it off. Goodreads has the following excerpt:
The clone pulsed up beyond the sink drain, and it was then that Maude Wendall saw it. She frowned with annoyance at the sink stoppage, and prodded at it with her pot scraper. It resisted. She tried to push the mass back down the drain, but it would not go. Clicking her tongue in exasperation, she dropped the pot scraper and prodded the mass with her finger.
Through the permeable cell wall of the clone's tissue flowed the enzyme-laden ichor. On contact with human tissue, the enzymes immediately broke down its proteinaceous structure and utilized the amino acids and other residues to construct new clone tissue. There was no pain in the finger. It was several seconds before the woman realized that her finger had disappeared, had been replaced by the clone's tissue. She screamed then, and lunged back from the sink.
The slime invades human flesh as soon as it touches you. Even the tiniest touch was enough, and it converts human flesh into its own material so it grows by absorbing humans. I mention this in case it jogs your memory - the slime contains less water than human flesh so as it absorbs people the excess water is shed as a spray of water.
I cannot for the life of me remember how it ends, but I think it ends with the clone just running out of energy and dying rather than by any human action.
Something worth mentioning in case you've mixed up two books over the years: there is a short story by John Wyndham called The Red Stuff about a red slime found on an asteroid in the Solar System. This is accidentally released on the Moon where it threatens to overcome the whole Moon. However this slime doesn't directly harm people. It kills only because it swamps people so the suffocate.