You raise an interesting question because we usually associate slugs with flowers or vegetable feeding damage. However, actually there are many species of slugs and snails and they eat all kinds of things, from dead plants, fungi, and dead animals to animal excrement. Most slugs are scavengers.

No cause for concern because there is a slug in the tree.  Though, you're right, that is one determined slug to get that high up. It could be feeding on the moss or some random fungi. 

Ellen

We don't have an in-house specialist at the moment but it would be common for gray slugs to grow to 6 inches. They elongate as they climb/reach and then kind of accordion tighter again before elongating again as they move.

Christine