You cannot activate "NEX" if there is no Fusion Monster in your Fusion Deck with the same name as a "Neo-Spacian" monster you control.
If "Neo-Spacian Dark Panther" copies the name of "Neo-Spacian Aqua Dolphin", it can be sent to the Graveyard for "NEX", but no monster can be Special Summoned from your Fusion Deck. "Neo-Spacian Dark Panther" is in the Graveyard, its name is no longer treated as "Neo-Spacian Aqua Dolphin", and there is no Fusion Monster whose name is or is treated as "Neo-Spacian Dark Panther".