yeah, those definitely look like worms. See if you can grab one and look at it under a magnifying glass or super zoomed in with a phone.
It could be that you're seeing just the dead worms? Maybe something is making them die so you're only now seeing them.
Usually if you have a full grown betta or fish in there they make tasty snacks, so you probably wouldn't see a buildup. The babies wouldn't be big enough to eat them or want to.
I mean...it could be snail poop? But I doubt that, it doesn't really float. Definitely looks like worms to me. The ones I have move veerrryy slowly. I don't notice their movement unless I really watch them.
The worms I mentioned tend to show up when there's an excess of food, so since you're feeding the babies so often that could be why you're just now seeing them.