Learn Croatian: Mahovina raste po starom deblu.

While na usually means "on" (touching a surface), po implies being spread or scattered all over or across a surface. Saying po starom deblu paints a picture of the moss covering the surface of the trunk, rather than just sitting on a single spot.

What case is starom deblu in?

It is in the locative case. When the preposition po is used to describe location or movement distributed across a surface, it requires the locative case.

Is there an alternative way to write the adjective starom in this sentence?

Yes, you might see it written as starome or staromu. These are alternative, longer endings for masculine and neuter adjectives in the locative case. While starom is the most common in spoken Croatian, the longer forms are often used in formal writing.

The verb raste (from rasti) is imperfective. What are its perfective counterparts?

To express the completed action of growing, you will usually see the perfective verbs narasti (meaning to grow up or reach a certain size) or porasti (meaning to increase in size or amount).