Can algae permanently stain walls?

For health reasons I was unable to care for my pool for a number of months. My wife was less diligent than I am, not placing any blame, and we had a pretty good algae build up before I could really give it some attention.

I've got it pretty well cleaned up but there are some areas where it appears either there is still algae present or it is permanently stained the color of the algae. I'm pretty sure it is the mustard algae, but when it gets really bad it does turn a greenish. The current stains/algae are a lighter yellowish brown.

Do I need to give it a longer shocking period? I only kept the chlorine really high for about a day, which in the past would clear up problems. Maybe that's not long enough for these tough patches?

Or do you think it is permanently stained?

Thanks!