Can i use mana from ekko legendary to pay accelerate cost for next unit

Yes — if you mean your Champion Legend Boy Who Shattered Time (Ekko), its cost reduction does apply to the Accelerate cost.

One correction though: the Legend doesn't give you mana. Its ability is a discount — "Reduce the cost of the next unit you play this turn by [2][A] and give it 'Kill me at end of turn.'" Accelerate is an optional additional cost, and discounts apply to the unit's total cost after additional costs are added, so the [2][A] comes off the base cost and the Accelerate cost together (Rule 356.4.f; same treatment as Kraken Hunter, FAQ #4836). A power-denominated discount like [A] can cover the power portion of Accelerate even though Accelerate's power normally has to match the unit's domain.

Two notes: you can't bank the reduction — it only applies to that next unit played this turn, and the unit still gets "Kill me at end of turn." If you actually meant Ekko, Recurrent's Deathknell ("Recycle me to ready your runes"), those are just ordinary readied runes you can spend on a later play — they don't float, and they aren't available in time to pay for a unit that's being played as part of the same trigger.