I've been cleaning them for some time. At first I thought it was some kind of synthetic material, but when I took one with my fingers it disintegrated into filaments. I placed another under my usb microscope and they appear to be an aggregate of green tubular bodies.
They grow (appear) from one day to another, only 1-2mm diameter. My best guess is that they are some kind of algae, but I'm lost to find exactly what. I live in the south of Spain, the pool is treated with chlorine, in a well illuminated zone. There are no other algae growing, and the area is urbanized.