Oren wrote, on Thu, 29 May 2014 07:44:46 -0700:

Possibly the algae treatment. I'm using one for a couple half-dollar
sized black algae spots on white plaster wall ( (I let the pool go a
bit over winter).

Heh heh... *you* let it go "a bit" for the winter?
Me too!

I figured I'd do my part to allow the "greening" of the waters,
in order to feed the birds and the bees, over the warm winter
here in California.

Here's a 'before' shot:
https://c2.staticflickr.com/4/3701/1...27d69c41_c.jpg

Do you think I can grow huckleberries in that green pool?