As you probably figured out already, this is the personal status message (PSM) field. This appears to be a bug in Butterfly as it doesn’t appear to save empty PSMs.
As a workaround to accomplish this until this is fixed, sign into Butterfly again and remove the text in the PSM field, then click on the text field with your name above to make sure it’s sent to the Messenger server. I’d wait about 30 seconds to make sure the server has received the new empty PSM and then sign out.
Next, press R (Windows Key-R) on the keyboard to bring up the Run box.
Then copy and paste the corresponding text to your version of Windows into the Run box and choose the OK button. If you do not know what version of Windows you’re using, just try both options separately.
For Windows XP
%userprofile%\Local Settings\Application Data\Butterfly Messenger
For Windows Vista, Windows 7, Windows 8 and Windows 10
Once the folder opens, delete the .mcl files contained in this folder. You can keep settings.mcl though as the PSM isn’t stored in that one.
Sign into Butterfly again and the PSM should be removed as it’s no longer stored on the server or in the Butterfly files.