Maralen, Fae Ascendant

If an exiled card has {X} in its mana cost, X is 0 for the purpose of determining its mana value.

Since you are using an alternative cost to cast the spell, you can't pay any other alternative costs. You can, however, pay additional costs, such as kicker costs. If the card has any mandatory additional costs, you must pay those.

If the spell you cast has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

If Maralen enters at the same time as one or more other Elves or Faeries you control, its first ability will trigger for each of those other Elves and Faeries as well as itself.