I cant tell if you have a salt water pool or not? If not I would say your CYA is on the high end for a non salt water pool.
From the sounds of it, it sounds like you need to shock.
pool-school/shocking_your_pool
To get your shock value use this table
pool-school/chlorine_cya_chart_shock
If non salt water your shock level is 24, if salt water your shock level is 24 as well.
So what I would do is put in enough bleach to bring your level from current 7 to 24.
The pool calculator is pretty easy to use
I assume you already have the link if not here it is
http://www.poolcalculator.com/
At the top where it says size put in your gallon size of 21,000. For each test you will see two boxes next to each other labeled at the top Now and Target. The now box is for your current result and your Target is what you are trying to get to. So what I would do is in the now filed in the FC section put 7 and target 24 and that will tell you how much bleach to put in. Make sure when you get it, that it is 6%. Before you add the bleach adjust your pH down to around 7.2. That will require 57 fluid ounces of 31.45% muriatic acid. Once you get this down and after about 30 minutes of adding that, I would then do your bleach. Dont worry about anything else, because bleach is what is going to kill that algae that it sounds like you have. So after 30 minutes of adding the acid then add the bleach. This will make sure you get your pH down a little, because any value for FC over 10 pH will be wrong. So you will want to try to test as often as possible and keep the FC at shock level of 24. The longer it sits below that the longer it will take to clear. Pending how bad it is, after 3-5 days you should start to see some clearing of the water shade starting to get more blue.
When it appears to start looking more blue instead of green then you will want to do the OCLT-Overnight Chlorine Loss Test
pool-school/overnight_fc_test
So hopefully that can get you started some.
