Is this well water?

If it isn’t showing up in water from a hose, and is in both hot and cold inside, it seems like it’s a problem with the line from the inlet. Not Sue how you get the landlord to address it though. I’d get a water test done and go from there.

@vinmarks nope, it's not well water

@Rachel Lee thanks! where is the line from the inlet located?

@Joseph Corlett, LLC thanks! are these valves located through the entire plumbing system or in particular areas where they can be checked? Do you know of a lab that can test unknown materials?

The inlet is where the water line comes into the house. You said the particles aren’t in water from the outside hose, so it has to be something inside the house.

@Pier Ave The reason why I asked if it was well water is because we get black sand sediment in our well water so we have to use a sediment filter.

I'd call an experienced plumber before Googling for forensic testing.