Send 1 'Elemental HERO' monster from your Deck to the GY when you resolve "Hero Mask".
"Hero Mask" targets 1 face-up monster you control.
If you select a Monster Token and send "Elemental HERO Neos" to your GY, you cannot return the selected Monster Token to your Deck to Special Summon a 'Neos' Fusion Monster.
If you select "Grandmaster of the Six Samurai" and change its name with the effect of "Hero Mask", and afterwards you Summon another "Grandmaster of the Six Samurai", when the effect of "Hero Mask" disappears in the End Phase the Summoned "Grandmaster of the Six Samurai" is destroyed because you cannot control more than 1 face-up "Grandmaster of the Six Samurai".
If you select a "Harpie Lady 1" with "Hero Mask" and send "Elemental HERO Neos" to your GY, the name of "Harpie Lady 1" will be treated as "Elemental HERO Neos" and it can be returned to the Deck to Special Summon a 'Neos' Fusion Monster.
If you select an "Elemental HERO Neos Alius" that is being treated as a Normal Monster Card and change its name to "Elemental HERO Necroshade" with "Hero Mask", if you Normal Summon the "Elemental HERO Neos Alius", so it becomes an Effect Monster, its name will be treated as "Elemental HERO Neos".