Completely clear. Tank is dark green water butt with lid on , in a basement so yes, in the dark.
I had a nose round the swimming pool, fishpond and aquarium forums and they talk about clear algae slime caused by bacteria - but that seems to be from animals and plants being present in aquariums and ponds and people in hot tubs and swimming pools.
Apparently it's harmless but doesn't feel good (in hot tubs and pools) to bathers. They then go on about clearing it with chemicals etc.
Have a nose around their forums. Maybe if it's only on the pump and cable give it a clean and not worry? Can you check if it is getting on the walls of the butt? I don't suppose it's the toughest of jobs to empty and clean a water butt.
My pump and pipes are external to the tanks so have no personal experience of this maybe someone else will be along with more help.