Aquariums: First my snails explode in population, now there’s are these tiny, white, slug like creatures, what are they?

rhabdocoela Is a small flatworm that is completely harmless to shrimp, snails, and fish. They play the same role as detritus worms in aquarium which is eating random scraps and bits of food that fall to the bottom of the substrate

Not to be confused with planaria, which has a arrow shaped head and grows way larger and eats baby shrimp