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

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.