Potential Bird Plucking

I am a bit concerned about my Pineapple GGC. Recently I think he has devoloped some plucking. Idk if this is something that needs action or I let my bird work itself out. I have ruled out disease because I have a Crimson Belly and it would have spread to him by now. I am not sure if he is stressed, because wouldn't they both be feeling stressed. They come out of the cage all the time, they only spend time in it to eat and sleep. I started to notice this like 5 days ago. Then again its just a few feathers that have seem to be gone. Maybe it will grow back. I am concerned and wanted to speak with someone who may be more knowledgable then me.