Nip along to any garden supplies shop and purchase some metaldehyde contact and systemic molluscicide (slug pellets) as these put down at floor level will attract the bastard and then kill it.
Once the slug has been despatched, may I suggest you invest in some damp treatment because slugs can only survive where dampness exists; otherwise more slugs will appear from the eggs left behind from the original offender.
Thanks, I know how it got in, it came in through the French windows.
No problem with dampness as I have an in /out weather station which informs me that is currently 46% humidity indoors and 85% humidity in the garden.
If you can't answer a man's arguments, all is not lost; you can still call him vile names
Elbert Hubbard Post may contain sarcasm.
Cad, bounder and part time scoundrel.