False Puffball
The person who took this picture is a lot braver than we could hope to be. Knowing that there was a living slug in their home, they decided to snap a picture of the snail and what it was eating. Don’t they know that slugs are dangerous creatures that could take over their home when left to their own devices?
Anyway, the OP knew what the slug was, but they wanted to know what it was munching on. Apparently, it was a slime mold known as a False Puffball. What a hilarious name for such a disgusting organism. Do you kill it with fire or just let it be?