What are the tiny creatures or insects that crawl over the surface of some slugs?

Slug mites are the tiny creatures that can be found crawling over the surface of some slugs.

Specifically, slugs can be hosts to parasites. The most common parasites include certain flies, mites, and nematodes. Slug mites (Riccardoella limacum) live in their host's respiratory cavity, from which they quite visibly swam the slug's body.