I'm no expert but have a good idea what the experts will tell you. With algae you need to maintain shock levels until the chlorine holds over night. When it does this, it means the chlorine isn't being consumed by anything in the pool and the algae is dead.

Even though you water looks good after a 1/2 drain and refill, you won't know if all the algae is dead until your chlorine holds over night. If it doesn't then you will most likely end up right back where you started if you don't keep up with the chlorine at shock levels.

You'll need to check the water a few times a day and add chlorine as needed to keep it at shock levels. Up and down with CL levels is a no-no.

Good luck.

Russell