Just having come through a massive algae attack, I am pondering why can't we filter live green algae, only the dead
When the pool was a deep British Racing Green I had my sand filter filtering 24/7 and in the morning backwash there would be a derisory amount of green algae removed, just enough to give me a mixture of encouragement and dismay. After hard shock treatment, with the water turning a pale grey-green, the morning backwash removed a ton of grey gloop. (I am now mercifully sparkly clear.)
So why isn't the green algae filtered to any appreciable extent? It doesn't get larger when it dies, does it? And it isn't that it is constantly being replaced, as more would be caught in the filter.
I did use cal-hypo to shock (bleach is another story) but I can't see that being the cause of the grey stuff.