Alma no longer has a copy of 'Mugen', since Alma's
proper (although unconstrained) Innocence has been revealed.
Debating whether it's parasitic or equipment, although currently assuming parasitic.
Since Alma is much older and Kanda's Innocence has changed forms from it's feathery sword-shape into a sleek katana, Alma's Innocence has been modified as well.
Currently saying that it deactivates to look like a white glove on his right arm--as opposed to Allen's puppet-like parasitic Innocence. Since Alma technically had an arm and wasn't born with the parasitic Innocence, it has attached to the skin and outside of the arm already there--like a glove. This doesn't mean he can remove the 'glove' however, since the Innocence has bonded with deep tissue under the skin as well as nerve ends.
When activated, it takes on a scythe-like shape, which Alma can slice through his enemies with. He fights in a style similar to Kanda's combined with Allen's style of claw slices.
Although it can shift and move, the Innocence normally has two twin scythes that come from near his hands, while another smaller scythe sprouts from about his elbow.