If my unit has 1 damage marked on it and my opponent plays elder dragon can I react with temporal breach hidden before it dies?

No — your unit is already dead before you ever get a chance to play Temporal Breach.

Playing a unit resolves immediately with no priority window (FAQ #2971; Rule 337.2), so Elder Dragon is on the board before anyone can react. The moment it enters, its passive makes your opponent's 1 marked damage lethal and the unit is killed in the ensuing cleanup — before any reactions can even be played to its "when you play me" trigger (FAQ #10686). Hidden granting Reaction doesn't help; there's simply no window to act in. This assumes that 1 damage was marked by the opponent, since the passive only applies to "your damage."