Algae Issue - please help!

Hi All,
We have a 3 year old 22K gallon salt water gunite pool and although the water is clear, there appears to be blackish/green algae present. There are some spots that won't come off the stairs, a few areas that I can brush around that cloud up but the underlying spots won't budge. The PH is high (off the chart) and according to the Taylor test I need to add 2.86 qts of Muriatic acid to bring it to an optimum level. Total Alkalinity is 170 ppm. Free Chlorine is 2.4 and combined chlorine is 1.2. Any suggestions on what I should use to eliminate the algae? We have a beautiful weekend coming up here in Mass and plan on using the pool.
Thanks to any and all suggestions!