marsite stains

hello everyone im newbie,here is my problem my marsite is 11 years old ,been well maintained but in the last 4 months the whole bottom has become "like if someone scattered black sand " on it and it wont brush away it is starting to go up the walls and steps too .i have no idea what it is any help would be appreciated

Black spots that appear in pool plaster, or "Marcite", are often black algae. You can check by seeing if you can scrape a bit off with a fingernail. If you can, smear it on a white card. If it is green when smeared, it's black algae.