Rust spots on stairs

[Randrx2]

Need some help please. I noticed rust stains on plastic stairs. Brushing the stains does nothing. Comes off with vitamin C tablets. But there is still a tiny black dot there afterwards (those are the specks you see in the pic). Almost like an embedded piece of metal. I put a strong magnet there, but nothing comes up. There is poor circulation there even though there is a return. Dirt always seems to accumulate on the stairs in the corners. Any thoughts?

FC=5
CC=0
pH=7.6
TA=50
CYA=50
CH=120
Salt=3400

 

[PoolStored]

Lawn fertilizer.

 

[Randrx2]

Lawn fertilizer.

Maybe. Could birds carrying it in? I do my own fertilizer and am pretty careful. The grass is far enough from the pool. Would it embed in the plastic like that? If so, how do I get it out?

 

[PoolStored]

Birds? Likely not.

It's just a classic look of fertilizer. The vit C usually takes the iron out. If there was something else in the fert that damages the plastic, I haven't seen it, or a remedy here.

 

[Randrx2]

My other thought was that there is like a metal fiber in the plastic steps binding it together and the plastic is wearing away exposing it.

 

[Randrx2]

UPDATE: Today I took a dental scraper pick thingy and I scraped the spots. They did come up. I still don’t know what the heck they are, but at least I know it’s not something embedded in stairs.