-- Momentarily camping out in the cabin owner's office --

I did miss the liner bit; if it is black algae, you can eradicate it from a liner pool. But . . . you must get all of it, and you should try the "shmear' test Watermom described to verify that that's what you actually have. In liner pools, plasticizer bleeding can look like black algae, but algaecides won't help at all.

Ben