Moldessa is a card good for mid to early game for destroying monsters with low hp. When placed it will do 2-4 damage to an enemy monster, if no monster is present it will do no damage. Used in control decks, particularly Justice. Decently useful in tandem with Same Fate or Asgore.
As of Beta 9.3, another good strategy is combining Moldessa with Moldbygg and Feast (and Moldsmal) to become very hard to be killed, because Feast will increase Moldessa's HP to 6 and Moldbygg will fully heal all Mold monsters at the end of your turn.