It was likely never gone. Respectfully, you
seem to be ignoring the advise of expert members here, who are telling you that you will not clear algae with a one time shock of chlorine. Clearing algae is a PITA process, but the TFPC method is the best way of doing it. I don't (yet) have a pool, but I've seen plenty of people go from algae to sparking clear water. That path is not an easy one, but
if you follow the advise here, it's a sure path. If you keep doing a one-time shock of the pool, you're going to keep struggling with algae.
The problem with a one time shock is it will kill most of the algae, but not all, and later on the remaining algae will come back to life when the FC level drops.
As has been mentioned by multiple people, you need to SLAM, not shock. SLAM stands for Shock Level And Maintain. You bring the FC up to the shock level for your CYA, and
hold it there until you kill all the algae. This
will kill your algae, but you have to be willing to follow the procedure through till the end. If you do not, you'll end up wasting your time and money and be back to step 1 with a pool full of algae.
This article outlines how to do a SLAM. Be sure to have plenty of liquid chlorine on hand before you start, you
will need a lot of it. You need to do a full set of tests, as people here have been asking you to do, especially the CYA test. You will have to test and add FC multiple times a day, as well as daily brushing, and probably cleaning the filter a lot as well.
And you have to keep this up until the end conditions listed in the article. This will likely take several days at a minimum, probably longer (weeks, at the long end), but again, it WILL clear your algae once and for all!
After that, if you
maintain your FC at the proper level for your CYA, you should never have to deal with algae ever again. That's the beauty of the TFPC method. I've seen members here with sparkling clear water that they maintain with ease year after year with no algae or clouding. Algae happens when your FC drops below the required level for your CYA, usually due to ignoring regular pool maintenance (which includes regular testing and addition of FC).
