According to the picture you provided, this bug is a "horseshoe crab bug", which is also known as "King Bass Fish, Water Turtle, King Bass Cover, Horse's Hoof, and Sinagan Knot Bug (in Mongolian)". It is a small crustacean belonging to the order Gastropoda, the dorsal armor. They are mainly distributed in Europe and North America, and the two species that have been found in China are the Chinese horseshoe crab worm and the abundant horseshoe crab worm.

Horseshoe crab worms have an oval body, a bulging back, a flat abdomen, and gills on both sides for breathing. Their life cycle consists of three stages: eggs, worms and adults. Eggs are the size of a grain of rice, the body of the worms is translucent, and the body of the adults is grayish-brown or dark green.

Although horseshoe crab worms are regarded as pests in some places, they are actually an important aquatic organism that plays an important role in maintaining the ecological balance of water.