Can Reksai, Breacher grant accelerate to a card played from Hidden?

Yes. Rek'Sai, Breacher grants Accelerate to a friendly unit played from Hidden, because a card played from its facedown state at a battlefield is not being played from a player's hand (FAQ #6897).

You still have to pay the Accelerate cost — [1] Energy and 1 Power matching the unit's domain (or [A] if it has no domain) — during the Pay Costs step, and this applies only if the hidden card is a unit (Rek'Sai's ability doesn't affect spells or gear). CR 805.1.b.