[Lisp] Highlighting question

[alex]

Hello,
I have a problem when I load lisp-files with Emacs. By the time I
start typing the text gets boxed in gray.
When I evaluate/compile the code in Emacs using Slime then the dark
background of the code disappears.
When I edit source files of another type (e.g.: c++) everything gets
highlighted as I expect it to. It is only with Lisp.
How can I disable this behaviour?

Kind regards,
Alex