Is this destruction different from sealing the spawn?
Yes, these are different effects.
Also, when sealing a spawn and there is a monster on it, it says to destroy the monster. Is this the same as the monster dying or is the monster being banished? The reason I ask, is that if it dies, the explorer recovers an essence, but in the case of a banish, they don't.
You gain essence. You don't gain essence by removing monsters, but you do with destroy effects.