A hat icon in the upper left corner of an avatar indicates that today is the user's birthday.
If you're happy to see the hat icon on your birthday, add your birthday to your account settings.
When your birthday comes, a hat icon will be displayed on your avatar. This icon will be visible to all users accessing the space, regardless of their role or organization.
How to hide birthdays
If you want to turn off your own birthday, you can delete your birthday information in your account settings.
Tips
When your birthday comes, will be displayed on the profile viewing screen. (The name of each user will be displayed in the name section.)