Maelstrom Muse — ruling 1

To determine the total cost of a spell, start with the mana cost or alternative cost you’re paying, add any cost increases, then apply any cost reductions (such as that of Maelstrom Muse’s triggered ability). The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was.

Maelstrom Muse — ruling 2

Maelstrom Muse’s cost reduction can reduce only generic mana in a spell’s total cost. It can’t reduce colored mana requirements.

Maelstrom Muse : règles, fonctionnement et interactions (MTG)

Maelstrom Muse is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Flying Whenever this creature attacks, the next instant or sorcery spell you cast this turn costs less to cast, where X is this creature's power as this ability resolves.

How it works : Maelstrom Muse

Original English text — Wizards of the Coast / Scryfall

#1April 16, 2021· Wizards of the Coast
To determine the total cost of a spell, start with the mana cost or alternative cost you’re paying, add any cost increases, then apply any cost reductions (such as that of Maelstrom Muse’s triggered ability). The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was.
No other card is named in this ruling.
#2April 16, 2021· Wizards of the Coast
Maelstrom Muse’s cost reduction can reduce only generic mana in a spell’s total cost. It can’t reduce colored mana requirements.
No other card is named in this ruling.