marsite stains

[haleo]

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

[PoolDoc]

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.