Thank you, thank you PoolDoc and Watermom! I really appreciate your responses.

Last fall I did the Borax treatment you recommended here. Although the strip test I have isn't terribly accurate I think it's around 50ppm. I will keep raising it though as we may have lost some due to water changes.

I've kept the chlorine levels around 20ppm and it's crystal clear this morning! Just have a ton to vacuum out now off the floor. Would you keep the pool at shock values longer? If so how much longer to totally kill the mustard algae?

I may switch to liquid bleach as I dont want to raise my CYA anymore. I'm just afraid I'll forget a dose!