When using direct address, (such as calling someones name) the vocative noun case is used. This changes names that end in "-us" to end in an "-e". I.e. Marcus to Marce, Stephanus to Stephane.

Marce is a vocative form. You used the vocative when you are talking directly to someone. All 2nd declension nouns (Marcus is a 2nd declension noun) take "e" as the ending in the vocative.

Salvete is used when talking to multiple people, while salve is used for 1 person.