Tiny snakelike larva or worm

What is this little larva or worm that swims in a quick, serpentine, sine wave pattern? It’s around 0.5cm big. It’s in dirty, stagnant water. I’d like to know about the other one too.

They are not mosquito larvae - I put BTI mosquito bits in there weekly (mosquito bucket of doom). Sorry I couldn’t get it more in focus. Location TX