According to Google, it is a Macrolepiota Albuminosa.
Assuming Google is correct, I did some research and it shows that this type of fungi is used by people worldwide and aids in preventing diseases for humans. It is safe for humans to eat.
I'm not sure if it would be safe for chickens, though.
Maybe @nuthatched can help?
I would personally get rid of anything that could pose harm to your chickens, just to be safe.
@All4Eggz
Hmm. It's hard to say unless a full view of the gills and cap, it's likely safe, with that size. But it's better to be safe than sorry with mush'ems.