Primary course is to first kill the algae. Allot of shock. If it's green, use at least 20-25lbs initially. Brush it in and let run 24hrs. Drop PH with muriatic acid to at least a 7.2. And, stock chlorine tabs in skimmers, floaters, etc to boost stabilized chlorine levels to at least a 3ppm.
Once run 24hrs, add a floc agent. Run pump another 6-8hrs, then turn off and leave off. Dead stuff will settle in about a day. Then vacuum to waste sediment. Might take 2 or even three vacs letting settle after each attempt. Keep water level high until completed.