Yellow gunite

[Sboyse]

Hard to pick up in pictures but I have a few spots (mainly on benchs) that appear to be yellowing. Will not brush off. Pool is just over a year old...I have been pretty anal about upkeep. Any idea what ot could be?

 

[JoyfulNoise]

Try running a sock of crushed vitamin c tablets over it for a few minutes. If it lightens up, it’s iron staining.