  The spontaneous muonium-to-antimuonium conversion is one of the interesting
charged lepton flavor violation processes. MACE is the next generation
experiment to probe such a phenomenon. In models with a triplet Higgs to
generate neutrino masses, such as Type-II seesaw and its variant, this process
can be induced by the doubly-charged Higgs contained in it. In this article, we
study the prospect of MACE to probe these models via the muonium-to-antimuonium
transitions. After considering the limits from $\mu^+ \rightarrow e^+ \gamma $
and $\mu^+ \rightarrow e^+ e^- e^+$, we find that MACE could probe a parameter
space for the doubly-charged Higgs which is beyond the reach of LHC and other
flavor experiments.
