We can't determine which species of earthworm these are, as many exist in Maryland (though most earthworms in this region are non-native) and they can be difficult to tell apart. They feed on organic matter in the soil and on the soil surface, such as fallen leaves and dead plant parts (old roots, etc.) and should not be dangerous to any other animals that we are aware of. As to why they are in the container of water, we can't be sure. Some worms can crawl/climb up surfaces for a surprising distance in wet weather and may have reached the rim of the pot and fallen in, unable to escape. Others may have been deliberately put into the water by a predator about to eat them, but maybe it was interrupted and left the area. In general, earthworms rise up into shallow soil depths or leave the soil entirely during periods of heavy rainfall. One theory is that they do this in order to breathe since oxygen levels can fall in soils that are completely saturated, but that might not be the only reason. (Earthworms absorb oxygen through their skin since they don't have lungs.)

There's no need to take any action apart from monitoring this water source for mosquito larvae if you keep it in place during the growing season. Mosquito larvae can be treated with very little risk of harm to other organisms by using Bt (Bacillus thuringiensis) granules to kill the larvae as they ingest treated water. How often to re-dose the water to keep killing any mosquito larvae that appear into the summer will be information the product label provides.

Miri