somebody made a fake icloud account under my account

They didn't create an account, they signed in to your account. Which means they know both your user id and password.

Change your password and delete the unknown user from your account.

You are viewing your own Apple Communities forum nickname, and your own forum-related configuration settings.

if you did not create this nickname, then somebody has access to your Apple Account, and created it.

You can View Profile to see when it was created, on the off chance the nickname creation was a while ago and forgotten.