Rosa marina is a small, rice-grain-shaped pasta, essentially another name for orzo (also called risoni). Despite the name meaning "sea rose" in Italian, the pasta has no connection to the sea or to seaweed; it is simply named for its tiny, rounded grains.
Rosa marina is made from the same semolina dough as other short pasta shapes and is used interchangeably with orzo in soups, pasta salads, and baked casseroles. The name is most often encountered in Italian-American communities, where "rosa marina soup," a chicken or seafood broth studded with the tiny pasta, is a well-known regional dish.
Because it is functionally identical to orzo, rosa marina can be substituted one-for-one in any recipe calling for orzo or another small rice-shaped pasta; it takes on the flavor of whatever broth, sauce, or dressing it is cooked in.