Yes, I read the question correctly. You asked for an answer, and I gave you an educated guess with the information that I had about your pool.

I know that it can gather so quickly because it does in my pool, as well as many others. When you vacuum the stuff up, chances are good that some of it dissolves into the water instead of escaping through the vacuum.....but if you don't have a sand filter, and aren't introducing sand, then chances are good that it's either airborne stuff settling out, or some sort of algae. If your CYA is 20, and you consistently keep your chlorine above 3, then it shouldn't be algae if your water is clear. Do you have any combined chlorine? Do you have overnight chlorine loss? If the answer to both questions is no, and you aren't introducing sand/dirt into the pool, then it's just everyday stuff settling out of the water. Vacuum it up, or stir it up into the water so that your filter can get it.

Janet