If you previously had a performer account on MyFreeCams and you close the account, the username is still archived in the system, this is why a new name is needed.
You may make slight changes eg, HairyMonkey = Hairy_Monkey
Clients and regulars will always find you again.
You make also get this email if you have selected a common name, eg Username: Jane.
This chances the username "Jane" is available is quite slim.