Tiny brownish orange spots in plaster imperfections

I acquired a pool when purchasing a home back in May of 2019. I have kept the water balanced since that time. The pool has been fantastic. There is one ongoing issue that gets worse over time. I cannot figure out what is causing it.

The plaster is probably ten plus years old. But it looks good. No spalling or cracks. However, there are these tiny imperfections in the plaster (they may be normal). The best way I can describe it is texture you would put on a wall inside your home. Tiny divets. Inside many of those divets, there is staining. Some is orangey brown (hard to see in photo) and others are a grayish color (easy to see in photo). It will not come clean with the vacuum. And shock does not seem to effect it. Every time I go up to 30 ppm, every stain is lifted. These remain.

I will say I have a yellow algae bloom about once a year, that I SLAM to control. That normally knocks it out.

Pool Numbers:

FC - 3 PPM
TC - 3 PPM
TA - 90
CH - 250
CYA - 25 PPM
Temp - 80 Degrees

I have tried putting a puck on the spot - nothing. I have tried all of the metal magic compounds in their test kit - nothing. I have tried vitamin C tablets - nothing.

Could it be an algae issue that has not been controlled? Is this normal aging of the plaster?

Let me know your thoughts when you get a chance.

Attached is a photo of my pool step.