What's this floaty stuff that looks like dust or fibers?

[MaloK]

Stop feeding them completely, for a month or more.

Watch how the shrimps will deal with them when they are going to die.

If they eat the deceased worms and seed shrimps, let it go until both are nearly depleted.

They are the apex scavengers in the tank. And as long as their digestive tracks contain something. You don't need to feed them at all.

 

[WhistlingBadger]

...or borrow a plakat betta from a friend and throw it in there for a few days.

 

[Nells250]

UPDATE - the little thingies appear to be GONE! In fact, there are also fewer seed shrimp. Alas... I am having an algae problem, too... I thought shrimp ate algae!