I think these are the infamous root-eating nematode worms which some
people refer to. They came out of the drainage holes of my D.
capensis pot some time ago, and started swimming around in the water
tray. Nasty little critters, ain't they? They've also turned up in my
U. prehensilis pot; every morning when I turn the lights on, they
turn their noses downward and head back into the soil.
Try lifting them out with a toothpick and putting them on a paper
towel to dry out; then roll it up and throw it away.