Emperor Mihail II

The last ability triggers whenever you cast any Merfolk spell, not just when you cast a Merfolk spell from the top of your library. It will resolve before the spell that caused it to trigger, even if that spell has been countered or otherwise left the stack.

If the top card of your library changes while you're casting a spell, playing a land, or activating an ability, you can't look at the new top card until you finish doing so. This means that if you cast the top card of your library, you can't look at the next one until you're done paying for that spell.

You must follow the normal timing permissions and restrictions of the cards you play from your library.