If your FC is 31 ppm then you can't accurately test pH. It will test high due to a chemical interference between the chlorine and the pH indicator. Your true pH is NO WHERE near that high and is probably in normal range. Wait until your FC levels drop to normal after killing the algae before even thinking about TESTING the pH again.
(Technical reason: High chlorine levels will convert phenol red to clorophenol red which tests a much lower pH range than phenol red so the purple color you are getting only means that your pH is somewhere above 6.8, which is the upper limit for chlorophenol red.)