Does Magus of the Moon affect lands that are already on the battlefield when it enters, or only lands that enter afterward?

Magus of the Moon has a static ability that reads: 'Nonbasic lands are Mountains.' Static abilities generate continuous effects that apply to the game state at all times while the permanent with the ability is on the battlefield. This is covered under CR 604.1 and 604.2, which explain that static abilities create effects that apply continuously rather than triggering at specific moments.

Because the effect is continuous, it immediately applies to every nonbasic land already in play the moment Magus of the Moon enters the battlefield. It does not 'wait' for new lands to enter. Any nonbasic land that enters the battlefield afterward is also affected as long as Magus of the Moon remains in play.

The effect changes the land's type to Mountain (and only Mountain), which means those nonbasic lands lose all their other subtypes (e.g., Forest, Island, Gate) and gain the Mountain subtype. They also lose any abilities printed on them, since type-changing effects override the card's text under CR 305.7. However, they keep any abilities granted by other sources, such as Auras or counters.

Concrete example: Your opponent controls a Breeding Pool (a nonbasic Island Forest) before you play Magus of the Moon. The moment Magus enters, Breeding Pool immediately becomes a Mountain, losing its Island and Forest subtypes and its ability to produce blue or green mana. It can now only tap for red mana.

If Magus of the Moon leaves the battlefield, the continuous effect ends immediately and all affected lands revert to their normal types and abilities (CR 611.2b).