Be careful. Totally draining a pool is often not a very good idea. Particularly if the water table is high. Your pool can literally pop out of the ground. A better option might be to do a couple of partial drains and refills. You can probably safely drain 1/2 to 2/3 of the water. Then, retest cya. If still too high, can do another partial drain and refill. I realize that you have a large pool and this may be a big expense waterwise. However, if you do get the cya to a more manageable level with some partial drain/refills, then we can help you get rid of the algae. Won't be necessary to do a powerwash.

If you do drain and refill, repost with a set of water testing results taken from your fill water. It will be easy to help you get it balanced. Have you done any reading on our sister site www.poolsolutions.com ?? Lots of good info there. Repost with numbers and we'll make recommendations for you to get started off on the right foot.