Earthworms, the whitish ones have been in the water long enough to start decaying. During rainy weather earthworms will crawl around and get into all sorts of strange locations such as watering troughs, rain gutters 2 stories up that have accumulated soil in them, etc.
Hookworms are pale white and less than 1/2 inch long, attached to the inside of the small intestine and aren't found in their adult form outside of the host's body as is the case with roundworms, whip worms, and tapeworm segments which can be found in the feces.
keep an eye out for scorpions and black widows. But the tiny ads are safe.