If the pH is still 8.4 after adding a gallon of acid, there is a more severe chemistry issue. Also be sure that the test kit being used is giving accurate readings. I would suggest brushing the pool and adding a floccing agent such as "Drop-Out" or "Drop-Down" which will sink all algae to the bottom of the pool. It will need to be then vacuumed to waste (this is sometimes difficult to accomplish with above ground pools). Some water will be lost during this process. After this is done, the water chemistry can then be re-balanced. If the pump/filtration system is not set up to vacuum to waste or "backwash," other options would be to keep the pH around 7.2, elevate the chlorine and filter out the algae cleaning the filter frequently. If all else fails, drain the pool and re-fill. Try this chemical dosage calculator to adjust chemicals.