Learn Russian: Муравей медленно ползёт на ствол дерева. - The ant is slowly crawling onto the tree trunk.

Ползёт is the unidirectional verb of motion (from the infinitive ползти). It is used because the ant is crawling in one specific, continuous direction onto the tree trunk at this very moment. If the ant were crawling around aimlessly in different directions, we would use the multidirectional verb ползает (from ползать).

Why is the tree trunk in the accusative case (на ствол) rather than the prepositional (на стволе)?

Because it indicates a destination. The ant is moving onto the trunk, so we use на with the accusative case. If the ant were already located on the tree trunk and just crawling around on it, we would use the prepositional case to show location: на стволе.

If the ant were crawling off or down from the trunk, what preposition would we use instead of на?

To express motion off or down from a surface, you use с (or со) plus the genitive case. You would say Муравей ползёт со ствола (The ant is crawling off the trunk).

I see дерева is in the genitive case. Could we just stack the nouns to say tree trunk in Russian?

Unlike English, Russian rarely stacks nouns together directly to create a compound concept. Instead, you use the genitive case to link them. Ствол дерева literally means trunk of a tree, which is the standard way to express tree trunk.