Stains in spa? why and how to clean them?

[gazerbeam]

Appreciate feedback as to why I'm getting these stains on the wall of the spa. And how best to clean them?

The pool walls are fine and the systems are all interconnected. So I'm confused but that's my baseline state

 

[Texas Splash]

We can ask @JamesW for more thoughts, but if the pool walls don't do it, I wonder if it has to do with the elevated water temps which we know will cause some items (i.e. scale) to react quicker. Before James replies, please tell us the following:
- Post a full set of water test results
- Are you on a well?
- How old is the heater?
- How do you chlorinate and what products do you generally use? Any odd pool store products?

 

[JamesW]

Try sanding.

 

[gazerbeam]

Just opening the pool after winter so I don't have water test results yet. I use test strips (don't yell) and bi-weekly take a water sample to my pool store to confirm reading and get guidance.
Here's what happened last year according to my log:

• Chlorine typically reads 1-2, sometimes down to 0 and have to add some shock. (I use less than 10 gals of shock across the 4.5 mth pool season - excluding what the pool company add on open and close, usually 5 gal). I just buy the shock the pool store sells. Nothing odd.
  • I also add two 3" chlorine tabs each week to the floating dispenser
• PH typically 7.8 - it does range up to 8.4 and down to 6.8. Pool company on occasion tells me to add a little ph-
• Alkalinity usually reads 120 by the pool company - my test strip always reads higher 180 - IDK why
• Hardness 200-250
• Stabilizer 30-50

I do have a propane heater - Jandy LXi. It's has to be 10 years old, if not older.

I am on a well - well water test results below