I have a small electrical kettle (it has a plastic case and metalic
heating element at the bottom). I have previously (1 year+ ago)
descaled it by putting cheap malt vinegar into it and then turning the
kettle on.

On returning from a 2 weeks holiday, I now find that every time I turn
on the kettle, there are tiny pieces of black debris falling out from
the metallic elements. I left the kettle dry before I went on holiday.
Surely can't be (just) lime-scale? I live in London (i.e. hard water
error), but normally put brita-filtered water into the kettle for
boiling. Other times, I may use half filtered and half tapwater.

What are these black debris? Are they toxic? How can I remove it
safely?