Hi all,
I have both Leeches and Planaria in the tanks, but from this
but the worm is black, with a sort of cloudy white head growth
it sounds like a Bloodworm pupae. I get quite a few of these in the Bloodworm bucket. They pupate at the bottom of the "pond" in their tunnel, and then the pupae swims up to the water surface, and the adult Chironomid midge appears, like something from "The Alien". This is the pupae.
These Chironomid midges are really common, so probably eggs were laid in the tank, and these then hatched, with most of the blood worms being eaten by the fish, but this one swept into the filter where conditions would be suitable for it to grow and pupate.
cheers Darrel