Green-marked areas on the map in the Skippo app indicate that the area is covered by Hydrographica's harbor guide charts. This means that the area has detailed and specialized nautical charts designed to provide you with comprehensive information about harbors, docks, and other relevant nautical details specific to that area. These harbor guide charts are a valuable resource for navigation and planning, especially if you're planning to visit or anchor in the area and need detailed information about available facilities and services.