Mulm

The stuff that builds up at the bottom of an aquarium, usually organic sediment or dirt.
Can also be used as a greeting or a funny exclamation of sorts

A: "Hey B, how are you doing today?"
B: "MUUUUUUUUUUUUUUUUUUUUUUUUULM"
A: "MUUUUUUUUUUUUUUULMMMM"

"Ew, there's so much mulm in my aquarium."