Yesterday my water and pool looked fine and my chlorine level was at 2 ppm in the a.m.. I added enough bleach to get it to around 5 (CYA 50). Early this a.m. we had some thunderstorms and a hard 3-4 inches of rain in a few hours. It came down so hard that the water was coming in faster than my overflow could get rid of it and I had to backwash some out.
The water looks like an off color light milky green which would seem to indicate an algae bloom. However when I tested the chlorine it was still at a 2 which seems strange for an algae bloom. So I put on my goggles to see what was going on and it seems like there is a thin coating of something on the walls and bottom of the pool. Whenever I've had algae before, I've never seen it happen overnight like that, I usually see it as patches of algae starting in the deep end. It does look a bit like a few times I've taken the cover off and seen a similar the color only it's not not nearly as dark of a green.
So it would seem like it is some sort of algae bloom, but if so, why was there still FC of 2. combined chlorine of .5 or less. The worst part is that I have a pool party this afternoon so I don't want to start a shock routine until tomorrow. Is there something that could have come down other than algae to cause this that anyone knows of. Should I at least shock it assuming it once since it should still be safe to swim in?