Inclusions and second phases are totally two different things. Inclusions are mainly oxides while second phases are usually intermetallics. There are basically three groups of 5XXX alloys: Al-Mg, Al-Mg-Mn and Al-Mg-Cr. The common intermetallic phases are:
(1) Al-Mg: Al3Fe, Al6Fe, AlmFe, Al8Mg5, alfa-AlFeSi and Mg2Si
(2) Al-Mg-Mn: Al3(Fe,Mn), Al6(Fe,Mn), Alm(Fe,Mn), Al8Mg5, alfa-AlFeMnSi and Mg2Si