Barnacles that were removed with a pressure washer. Muriatic acid will dissolve the last traces of them, when finished soap and water will neutralize the acid. I use to do air conditioning cooling maintenance on a private yacht, removing growth from the system which included barnacles. It required redirecting the salt water intake into a recirculating loop with a 5 gallon bucket of fresh water, once circulating I added 4 liters of muriatic and let it run for a few hours. Just proceed carefully with common sense using appropriate protection and not around where pets could become contaminated.
