Clear water, but tough algae on rough spots

My plaster pool is eight years old and has some rough spots. I have algae that is growing in these rough spots and is a real pain to brush off. Using my normal pool brush on the pole doesn't do it - I have to use a hand brush and scrub, scrub, scrub. Of course, getting the spots below my arm reach takes many, many dives. It seems like I can never get it all. Any tips, or do I just have to keep working it? Shock it to 20 ppm Cl (it's 30K gallons, so that means tons of bleach)? Free Cl has been running around 5 and I have been adding poly, but that doesn't do anything for these spots.

Are you sure they're algae? Does your pool hold FC overnight?
If no FC loss overnight, then fairly certain its not algae- FC kills algae day or night, but gets used up doing so...
If it is algae, shocking is best way to get rid of it- get pool to shock level based on CYA level and hold it there til you pass overnight test (<1 FC drop between sunset and sunrise)