I have been dealing with this issue for the last two years (on and off).
Our pool is going on our 3rd Summer and the first year was great. Ever since I have had some black algae issues and luckily with all the good info that I read here on the forum, I finally got a handle on it (black algae).
My problem is now that the water is clear and looks pretty good, but when I brush the sides of the pool I get this major dust that comes off the walls and is especially bad in our grotto/cave where we have bench style seats. If I brush the walls at all it clouds the entire pool to the point that I cannot see the bottom and is a greenish color.
Our pools is a sculpted cement pool that looks like natural rock. The sculpted cement goes all the way to the pool floor and looks really nice but it has a pretty rough surface. Some parts of the walls have a fairly rough texture and the bench seats in the cave also are the same style and it is a bumpy surface. The surface is similar to a real rock, but a bit rougher with some 'pockets' that are probably 1/16" deep or so and it makes a nice hiding spot for all the 'dust'.
I have tried to use a vacuum hose (from a skimmer) to catch some of the 'dust' while I brush it but it is pretty much a losing battle
I think it is actually a fine enough powder that it goes through the filter and comes back through the returns? but i'm not sure.
I have tried to look at the captured 'dust' under a microscope but I don't have a good one and it is not clear if it is a plant form or solid.
Our pools surface during the building process is poured and sculpted, sealed, than plastered and stained. Before staining they sprinkled some 'greens' and 'oranges' powder to color the cement and make it look more like real rock. (think natural swimming hole in a river).
The walls when they have a lot of the 'dust' on them, I can run my fingers down the walls and my fingers have the 'greens' and 'oranges' and browns coloring on them? It does feel slimey?
My pool builder also told me NOT to use acid in the pool...but i'm not sure if the acid could eat through the 'coloring' or not.
If I let the water in the pool settle for a few days it comes back to clear again, sometimes up to 3 days, and it doesn't seem to be using a bunch of chlorine on a daily basis. Daily use has been up to 3 ppm per day (I usually have it at 7-8 ppm) and lately it has been using less and less and for the last week or two has only been losing maybe 1 ppm per day.
My test #s from earlier today:
water temp 80
I am at my wits end and am about ready to turn it into a coy pond
Any help, opinions on this would be much appreciated.