There are 2 types of algae which attract to swimming pools, mainly marbelite pools but its not uncommon in fiberglass either.
These are green algae and black algae. They both can be removed but once your pool gets the black curse its a headache to get rid of it.
What causes black algae? This is most common in pools which are heated, salt chlorinated or the pool is in a environment where its humid.
Now one can remove black algae by using chemicals and scrubbing the pool for a few hours but sadly it will come back. The best solution is to acid wash your pool.