I was unable to get a sample of the stuff. It vaccuums up very easily, and doesn't act like any algae I've ever seen, but I do suspect it's some form of algae.

Wierd, my CL levels and use rates haven't changed, I'm still getting the same readings. My PH has crept up to over 8, so I need to knock it back down to increase my CL effectiveness. Perhaps that's why it was able to get started.

We've also had almost zero swimming activity in the pool for the past two weeks, which is never good for ultimate circulation.