Spotted this long worm with a flat, shovel-shaped head slithering on the patio stones. It looks totally unnatural. What is this?

4. Origins and Spread of Hammerhead Worms
Hammerhead worms come from Southeast Asia but have become invasive in many regions, including parts of North America and Europe. They are usually introduced accidentally through the trade of plants and soil, since they can survive in damp potting mix during transport.

Once they arrive in a new area, they spread quickly. Their ability to reproduce by fragmentation means that even a small piece of a worm can grow into a complete new individual. Combined with the absence of natural predators in many of these regions, this allows their populations to expand rapidly.