What color are they? White/yellow-white - dead worms
Do they live and move and move with the water flow?

They are white and they seem to be alive

I found many small red worms tangled in the sludge collected on my pond filter cloth, below my waterfall. Totally submerged, but in highly oxygenated water. I put them in an ice cream box, with bedding and food and they grew in size.I'm pretty sure they are Eisenia fetidae, but don't know if this species is native to South Africa (although they are sold commercially). Anyway, they did well for six months until I got tired of feeding them seperately and so I dumped them into my main worm bin, with the red wrigglers.

Here is a link that might be useful: Eisenia fetida

I found many small red worms tangled in the sludge collected on my pond filter cloth, below my waterfall. Totally submerged, but in highly oxygenated water. I put them in an ice cream box, with bedding and food and they grew in size.I'm pretty sure they are Eisenia fetidae, but don't know if this species is native to South Africa (although they are sold commercially). Anyway, they did well for six months until I got tired of feeding them seperately and so I dumped them into my main worm bin, with the red wrigglers.

Here is a link that might be useful: Eisenia fetida

I often find grayish worms in my swimming pool - especially in warmer rainy weather. They survive for a while, but if I don't rescue them, they die fairly soon. I'd guess they were ordinary earthworms, rather than red compost worms.