Learn Finnish: Sinun täytyy ryömiä tänne hiljaa.

Finnish has a three-way system for location words based on movement. Because the verb ryömiä (to crawl) involves movement towards a location, you need a "to where" word. Tänne means "to here" (into the general area around the speaker). If you were already crawling around in one spot without a destination, you would use the static "where" word täällä (here).

Why is "you" translated as the genitive sinun instead of sinä?

The verb täytyy (must / have to) is a necessity verb. In Finnish, the person who "must" do something is always put in the genitive case (ending in -n). The verb täytyy also stays frozen in this third-person singular form, so the person doing the action is only shown by the genitive pronoun: sinun täytyy (you must), minun täytyy (I must).

I have seen hiljaa used to mean "slowly". Does it mean "quietly" or "slowly" here?

It can mean either, depending on the context! Hiljaa is an adverb that translates to "quietly", "silently", or "slowly". In the context of sneaking or crawling somewhere, it usually means moving without making a sound, but it naturally implies moving slowly as well. You will also often hear Hiljaa! used as a command for "Be quiet!".