By Law, Muslim is the person who says, Allah (swt) is one and only He is worthy to be worshiped and Prophet (sws) is His servant and messenger. People can be Muslim just by name and by birth too. Such people might not be Momin but they are Muslims.
By Faith, Muslim is the person who submits himself totally to Allah (swt), Muslim seeks Allah (swt) will instead of seeking his own desires. These are Muslims in the sight of Allah (swt) and Allah (swt) calls them Momineen.
Can you explain more about what you mean here. I know we are all born muslim, but whats this muslim by name malarky ?