of course he can, that's like saying "if Kabuto releases Edo Tensei would he be able to un-summon Itachi?" the fact is Itachi was un-summoned when Edo Tensei was released, Madara would be un-summoned too if he didn't know the seal to break out of it, so if Hashirama doesn't do the seal to get himself separated from the summoner, he will be un-summoned.