Dave, first .. welcome to TFP!
Here at this forum, we pride ourselves on assisting folks as much as we can, but also do so with reliable test results as more times than not, pool store testing is incorrect at best. So as a new member, the first thing we would recommend is obtaining either a TF-100 (link below) or a Taylor K-2006C test kit so we can see what your water results are like. I know, not the first thing you wanted to hear, but it's very important.
For stains, we need some history. How old is you plaster? Have you added any pool store chemicals to the water? Be honest!
What about iron in your local water supply? On a well perhaps?
Finally, the first two things we generally recommend is to check a stain for is iron or organic staining. Try the following:
- Place a chlorine puck against the stained area for a few minutes. If it lightens, it's algae-related.
- Place some crushed vitamin C tablets in a nylon or similar material and lay it there for a few minutes. If it changes, it's iron.
Also, please update your signature with all of your pool info. It will help a lot later. Let's start there and do what we can. Welcome!