If it's just underneath the text...

Right-click in the window where the mud sends text..go to world configuration

Click Triggers to highlight it
Click Add
In trigger put: you no longer can see invisible
in send put: Don't see invisible!
in Send to click and put: Output
Click OK until all windows are closed.

Piece of advice in trigger i'd put something like * no longer see invisible

On the off chance the prompt is sent as the same time as that message it will still fire if you do it that way.