My water chemistry went out of whack while I was not paying too much attention to testing, as well as "stationary" pool shark cleaner while waiting for replacement pods, feet and wings.
First signs were black/brown discoloration of the stainless steel light ring and black algae spots particularly on steps and near bottom of walls.
Added algaecide which helped a little for a few days and noticed that the algae started coming back. Added a second dose (making for a years supply according to the manufacturer - in just a couple of weeks.)
Also cleaned out the filter and vacuumed the pool only to note the return of the algae.
Testing showed FC =0, PH =8 etc.
Placing Pool pilot in boost mode generated a "clean cell" warning.
The cell was found to be completely clogged and took several iterations of 4:1 acid to completely clean it.
The water is now balanced, the algae is gone, but the discoloration of the light ring remains.
It is not removed by brushing with a stainless steel brush. Close examination shows it to be brown in color.
Can someone please tell me what is this discoloration and how do I get rid of it.
For what it is worth, the light may be removed from the pool and placed on the deck for testing or treatment if that is considered easier than treating the pool water.
There are also a few brownish patches on the floor of the pool which are rather unsightly given how sparkly the water is right now. I will try to post a picture of those patches later but hope for help with the light ring in the meantime.