This wiki lists Murakumo as an alias for Amagiri, but doesn't list how he gets it. I don't think anyone was calling him that before his high-profile duel with Toudou Kirin, but a girl calls him by the name at the start of the following episode.
I think it makes sense to assume that he was given the nickname when he took the rank 1 spot, but I don't know why that particular nickname was chosen for him. The wiki translates it to "Gathering Clouds" but I'm unsure what this has to do with anything. Is the reason for this nickname explained somewhere in the series?