Metal reactivity with hot water VS HCL
you might as well check the reactivity series.There is no unique and fully consistent way to define the reactivity series.The reactivity series is sometimes quoted in the strict reverse order of standard electrode potentials, where it is also known as the "electrochemical series":according to the series which has been defined there is no reason behind it.It is experimentally defined and according to the reactivity series magnesium is placed above aluminium.You might as well check this link for more clarity https://en.wikipedia.org/wiki/Reactivity_series#Reaction_with_water_and_acids.In your question it is given that magnesium doesn't react with hot water but it does actually vigorously. 
 Magnesium  reacts very slowly with cold water, but rapidly
in boiling water, and very vigorously with acids.Aluminium reacts with acids and steam but less vigorously than magnesium.Magnesium DOES react with hot water even more vigorously than aluminium . Please check the link given.
Yes another factor is the formation of oxide layer on the surface of aluminium which reduces its reactivity