I've found them in the flourite in a couple of tanks before.
they live deep in the gravel.
reddish, about 1-2" long, about the same number of bristles.

They are polycheates, but I'm not sure beyond the order.
There are few resources, I might bring one over to the nematode lab, they likely have some info on polycheates or at least some ref books.

I found nothing eailer when I saw one.

They do not appear to harm fish and most fish and plecos etc, don;t seem too interested in eating them either.

Regards,
Tom Barr