Your one and only pest!
Looks like some sorta Lovecraftian horror that's waiting to feed and get bigger like a baby Shoggoth. Or maybe a tumescent growth of William Burroughs' parasitical "undifferentiated tissues" waiting for a host to leap onto and infect.
Either way I'd run screaming from the house if I saw that coming up in my toilet! So the worms just stay in a clump? I'm not saying that's not what it is but it's so weird that it all pulsates at once like a single organism like that.
One of my uncles is on the water dept, if I think of it when I see him I'm gonna ask if he's ever seen something like that when he's been down in the sewers. He had to rescue two teenagers who thought they could climb into the sewers through a manhole with no breathing equipment and make their great escape... Nope! They brought up one dead and one almost dead from prolonged exposure to sewer gasses. So don't any of you get any ideas to go spelunking to find some gross worm colonys!