#DIYplumbing #mould
Please advise.
My other half has an apartment and AquaTrust visit frequently, to test water temperature (and possibly quality?) in the 2 hot taps, from a wall-mounted electric water heater. Is it a sign they have an open cold water tank, or some other potential issue such as Legionella?

And what's the best way to identify and/or get rid of the black stuff I find on the underside of the bathroom metal tap, where there is a plastic insert? I wiped the tap with a tissue and took a picture of the gunk, which I only spotted because one day I saw some wash away as I ran the tap.

To me, either black mould is growing on the plastic*, or something fibrous is getting into the water and building up.

*I'm think the tap is metal, and probably contains some copper, which would tend to discourage microbes.