Slug, or land slug, is a common name for any apparently shell-less terrestrial gastropod mollusc.
The word slug is also often used as part of the common name of any gastropod mollusc that has no shell, a very reduced shell, or only a small internal shell, particularly sea slugs and semislugs (this is in contrast to the common name snail, which applies to gastropods that have a coiled shell large enough that the animal can fully retract its soft parts into the shell). (source)
Can guinea pigs eat slugs?
No they can’t at all. They won’t try and eat them and will not find them particularly pleasant to eat, if they happen to do so.
So if you find one in their cage, then there is nothing to worry about. Just remove it and clean up their entrails.
For more things that guinea pigs can and can’t possibly eat, check out our guinea pig food list