So that is the little white thing in your tank?! The first picture looked like fish fry with little eyes but the close up definately does not look like fish fry or planaria. Actually the first thing that came to mind was baby squid...lol. Not sure they even look like anything mentioned so far. It does look like they have eyes, lung/gill (pink), and stomach (yellow-brown).
How do they move again? And what was the last thing you added to the tank before you noticed them (plant, decor, fish..etc)? Can you see antenna or legs?
Here is a good picture I found of an adult scud...possibly?
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