Hi Lucius. You may be our first member from across the Atlantic!
Those spots will come off very easily with a pressure washer. There are several threads on here too about different types of cleaners to use on Roplene such as this thread:
Can't get these tiny black spots off my boat
Use the search tool in the left hand column on the main page to enter "black spot" or "cleaner" to see all the threads you come up with. Have a great Thanksgiving holiday over there! Glad you've joined us.