I'm going to say it's a permanent stain.
I suggest you switch ot muratic acid for pH control. No mixing, pour straight from the jug.
Here is a post that talks about using dry acid and the sulfates that build up in the water from it's use.
Where do I start? I do miss the days of being blissfully ignorant on pool maintenance. Since swallowing the "red pill" on this site and diving deep into water chemistry, I find myself caught up reading various viewpoints and threads for hours on end. Watching people come into the pool store...
www.troublefreepool.com
Now, using muratic acid -
Safety first. Wear old clothes and eye protection.
I like to get in front of a return on my hands & knees and splash a little water on the pool deck. Then open the acid bottle and partially submerge it in the water. Tilt the bottle to pour the acid in at close to water level, but don't let the bottle get submerged.
I don't measure what I pour because it's an additional step for me to spill the stuff.
Remove the bottle from the water and set it down in the puddle on the pool deck while you replace the cap.
Try not to breathe the fumes..
Guess a little low on amount you need, then test again after it circulates well. You can always add more.