Compatibility of aluminum fittings with brass rough-in valve
It will corrode quickly. The Aluminum will break down.I recommend that you put a non-metallic barrier between the two. If you want to do it on the cheap, and If you have some Never-Seez or even some PTFE tape laying around, do it. Check it in a few months, replace the barrier, check again later.

Galvanic Corrosion happens because of the different in electrical potential. The amount of difference (and the environment) that determines how fast one metal will corrode. 


Aluminum has an anodic index -.9
Brass has an anodic index of about -.4


A difference of .25 is acceptable, in harsh environments (heat, salt water) the max is .15. 

http://www.fastenal.com/content/feds/pdf/Article%20-%20Corrosion.pdf
https://en.wikipedia.org/wiki/Galvanic_corrosion