I have an older inground pool with vinyl liner. I have growth that starts as a yellow-brown dust and then spread to to have green under the original dusty spots. The growth is only on the horizontal or sloped surfaces of the pool, not on the sides, and it is in shade or sun.
Last week I went to 20 ppm FC (by estimate) with 11 bags of cal hypo shock. The chlorine level was to 3 the next day and zero the day after. I realized that I had no CYA.
Thursday(?) I added 6 pounds of CYA (sock in skimmer) to try to get to 30 or so (yellowish brown on test strip, not quite brown). I brushed the pool. I added shock to get to 12 ppm and it held above 10 on the test strips for three days. Yesterday it was around 5. The pool has stayed green and cloudy since brushing and now I have growth on the pool floor again.
Yesterday I added 10 bags of cal hypo shock. This morning it looks a lighter green but has visible powder in the same accumulations on the floor. Anywhere there are wrinkles in the liner gets an accumulation first. If I vacuum the pool then those accumulations are removed with a green spot where the dust was. The entire floor of the pool is covered with a yellowish brown dust.
This morning I vacuumed the accumulations and brushed the entire pool.
I have never had this much trouble with the pool. We moved here 6-7 years ago. Last Fall I had a broken pipe above ground. It didn't get fixed for a couple of weeks. After fighting the algae then I drained half way and refilled with city water. I has been normal until the past two weeks.
My current numbers are
pH 7.8, maybe 8.4 by color
CYA is not making the brown square fully brown, so guessing 30
the FC indicator is dark, dark purple
the last picture is what I see when I brush the flat surfaces