Black Algae Struggles

[ejcom12]

After learning that the spots around my rock waterfall may be black algae, I completed a SLAM. I believe I passed the OCLT successfully, so I returned the water to normal chlorine levels. But I'm not sure if I was able to treat it all. Here are a couple of pictures from around the rocks after lowering the water (I needed to bring down the CYA from 65 to a more reasonable range so I drained about 1/3). Does this look like the algae survived and needs to be retreated? Or is this just remnants of dead algae?

What would you do next? Should I leave the water level lower until I'm able to fix this completely?

 

[Rancho Cost-a-Lotta]

Can you confirm these areas are usually underwater?

Are you able to scrape any up with your fingernail or another small tool? If so, black algae will smear green on a white piece of paper.

 

[ejcom12]

Yes, these are typically underwater.

I'll give it the smear test tomorrow in the light of day and get back to you. Thanks!

 

[ejcom12]

Yes, these are typically underwater.

I'll give it the smear test tomorrow in the light of day and get back to you. Thanks!

My apologies for posting and not following up.

To help others that might find this thread- A local friend gave me some advice to rub the areas with a chlorine puck while leaving the water low. After rubbing, I left the residue on top of the algae stains for about 30 minutes before rinsing with the hose. Cleared it right up!