They sound like 'detritus worms' to me (I don't know the proper species name for them). Planaria tend to crawl around on the glass, rather than being free swimming, and they look like tiny slugs rather than wriggly worms.
Detritus worms are harmless, but they are a sign that you're overfeeding, and that your maintenance regimen could do with being upped a bit, so more gravel cleaning for you, I'm afraid
To be honest, in fry tanks, you should be doing water changes at least every other day but preferable daily. Livebearer fry produce hormones that stunt their growth once they reach a certain concentration, so you need to be diluting that or they won't grow properly. And fry tanks are almost inevitably overfed, as you need to make sure they have enough food, so the extra water changes will reduce the problems caused by that as well.