Momo, Friendly Flier's second ability only reduces the generic mana cost of the spell. If first non-Lemur creature spell with flying you cast on your turn doesn't have generic mana in its cost, the cost will not be reduced and the cost reduction will not apply to the next spell you cast that turn.
Flying
The first non-Lemur creature spell with flying you cast during each of your turns costs less to cast.
Whenever another creature you control with flying enters, Momo gets +1/+1 until end of turn.
less to cast.