The scene you describe features Fry, off camera, falling into the sewer. However, not all sewer water is mutegenic ooze. Only the sludge in the town will instantly turn you into a mutant if you aren't one already. Regular sewer water isn't enough. And Fry was simply falling into the plain-old sewer, where normal waste-water has yet to become mutant-producing.
Initially I was going to say it was only the glowing ooze, but upon re-watching all the related scenes it seems that glowing isn't the required attribute. The glow was a temporary condition that eventually went away, long before the first attempts at changing anyone into a mutant. It was introduced by Bender when he dumped tons of radioactive waste into the sewer.
For some more evidence, please watch Teenage Mutant Leela's Hurdles. There is a boat race there which almost certainly got sewer water on Fry. If all of the water was mutegenic, he would have been mutated then.
Legion600 brings up another good point. We've never actually seen anyone mutate. Although Leela did use her own lack of changes as evidence to come to realize her own mutant status.