This card’s effect that negates Spell Cards that target it is not optional.
You can Special Summon this card from the Graveyard with "Monster Reborn" because its effect that negates Spell Cards is not active while it is in the Graveyard (but only if was properly Fusion Summoned originally).
A Spell Card that targets this card may be activated, but it will be negated and destroyed.
The effect of this card will not negate "Creature Swap", "Fissure", "Smashing Ground", "Dark Hole", "Raigeki", or similar cards that do not target.
If you try to equip "Fusion Sword Murasame Blade" to this monster, it will be negated and destroyed.