Rust-color on Prime Agra

[VickiC]

Hi,
Two of my orchids had a "whitish" fertilizer buildup on the Prime Agra surface, so I just finished going through the process of unpotting and cleaning the LECA (boiling, etc.). I also have another plant with some "rusty-color" surface Prime Agra (pic attached). Can somone tell me what this is, please? Is it a fertilizer build up, as well? If not, what? Is there something that I should do to remove it (if poss.), or is it normal and ok?

I would appreciate your advice,
Vicki