I've seen this before. I've got so darn many of them on some tanks that sometimes it is the only clue as to which fish has dropped her fry.
If you're seeing what I see sometimes, it looks nothing like any of the worms I've seen.
I personally think it is some sort of after-birth material that the females jetisson after finishing. It is pale white, stretchy and can get an inch or more long.
(No, it isn't the same as the white stringy poop when they've got bacterial problems etc).
I usually see this after they've finished. But I've seen it part way through on some females that I had in nets.
I've opened up females that have died (to see if there were any rescuable babies inside). The babies develop inside some sort of sac of material a lot like the stuff I'm talking about. I'm wondering if this is where the material comes from.