Maelstrom Nexus

This effect takes into account spells that were cast earlier in the turn before Maelstrom Nexus entered the battlefield, including any spells still on the stack. If you've already cast any spells that turn (including Maelstrom Nexus itself), this ability won't give any of your spells cascade that turn.

If the first spell you cast in a turn already has cascade, both cascade abilities will trigger separately. Deal with them one at a time: First one cascade ability will resolve, and you'll cast the applicable card if you want. That new spell will resolve. Then the other cascade ability will resolve in the same way. Finally, the original spell will resolve.

Cascade triggers when you cast the spell, meaning that it resolves before that spell. If you end up casting the exiled card, it will go on the stack above the spell with cascade.