  When the Standard Model Higgs sector is extended with a complex singlet that
breaks global lepton number symmetry spontaneously, a massless Goldstone boson
called the Majoron J arises. In addition to increasing Higgs invisible decay
through mixing, the Majoron can generally have flavor-changing interactions
with fermions. We find that type-III seesaw model poses such interesting
properties with both charged leptons and neutrinos. This opens up new channels
to search for the Majoron. We use the experimental data such as
muonium-anti-muonium oscillation and flavor-changing neutrino and charged
lepton decays to put constraints on the couplings. As a novel way to reveal the
chiral properties of these interactions, we propose an experimentally
measurable polarization asymmetry of flavor-changing l to l' J decays.
