Just saw the photos -- so-called black algae.

And, with an eroded pebble finish like that, you've got a long term maintenance issue.

Could you mail larger versions of those photos to me at poolforum AT gmail DOT com?

I'd like to edit them, comment on them, and repost them here. I can explain a bit more of how and why, and what to do, if you do (BLACKMAIL, here! )

Ben