Moldy spots on tail feathers

[AmyBella]

I was holding one of the girls today and noticed that she had little black mold / mildew looking spots on her white tail feathers. There is no mold or mildew in their environment, nor is the coop or run wet. Should I try to wash it off? Using what?

 

[canada-goose]

Maybe you should cut off the moldy feathers. I have never heard of that before but it doesn't sound good.