Hi. I have investigated this phenomenon in the past, and had no success in directly finding the cause. However, I did formulate a hypothesis.
The mucus seen is generated by certain cells in your body, but the genes responsible for production are likely promoted through a foreign source.
The mucus "type" is often seen in those with CF.
The black strands, or "fibre" you are observing 'may' be related here.
I could get into some more bizarre theories about this, but it is complicated and opens the door to a huge area of speculation.
My own personal opinion; treat it as though it is a fungal source.