One of my fundamental rules is that you should never, ever look inside the water lines serving your faucets. Having recently replaced a water heater, I had to violate that rule and discovered this growth inside the flex tube at the hot water outlet:
Anything that can thrive in 120 °F water gets my grudging respect.
Scrubbing it with a toothbrush produced gray sludge that rinsed right out, so I guess it’s all good now…
The Smell of Molten Projects in the Morning 
And if you do, never, ever show your wife.
She thought it was interesting… she’s a tough lady! [grin]
Did your interest extend to determining what kind of beastie that was?
Searching on the obvious keywords shows you can do science on them!
More than that, I cannot say… [grin]