How to recognize black algae in vinyl pool

[kkpoolnj]

I have a vinyl inground pool of 20K gallons. I opened it 2 weeks back. It had green clouds and dirt but got over it by shocking the pool. On 16th May i tested the water at Leslie's lab. Free Chlorine at 2.25. Total Chlorine at 3.05. pH at 7.2. Total alkalinity at 59. Calcium at 132. Cyanuric acid at 50. Iron 0.1. Copper 0. Phosphates 892. TDS 400. Added 10 lbs of Alkanility-up. Test strip shows all fine. Water is crystal clear. However 2 days back i suddenly saw lot of dark and bright black spots at the pool bottom. It could be easily moved by a brush. I vaccummed using a skimmer sock and lot of debris collected in the sock. There was even a piece of dirty black cloth (2 inch x 2 inch). So i thought that some bird may have have dropped something. This was yesterday. Now today again i see similar black spots. Much less in quantity but they are there. They are not concentrated in one point or area. They are almost all over. Is this black algae! Why does it feel like drime and dirt in the sock. If it is not algae and dirt; why did it come back. Why did the filter not stop it since these seem to be large particles. Please advice me on these questions. Iam worried if iam dealing with black algae in a vinyl pool here.