That's assuming that just because Gren lacks most of his original body, his aura is suppose to be weaker. This isn't Star Wars where cybernetics are inherently conflicting with mechanical parts or even armor. Besides that most of his aura already goes into making the machine work anyways, and that isn't limited to just allowing him to move. It effectively pumps his aura throughout his entire body like a mechanical heart, precisely so he doesn't need to constantly focus on pumping his spiritual blood all the time. With the benefit of doing what aura is suppose to do anyways, like protecting its person. 

Gren aura is constantly active for both protecting him and allowing him to live much like Sapphire is constantly breathing. Because as discussed before, Grens aura is what is powering his body (since we also nixed the idea of alternative energies like dust or fusion cores). We also had mentioned that while EMPs are effective against Gren, it's not instantly lethal, only that it shuts down a vast majority of his functions beyond what's keeping him alive. I.e. Moving at all.

The idea that he's constantly struggling to make his heart use his aura to just keep him alive is frankly kinda rediculous, not to mention making him much more of a liability If he can't rely on his aura to serve a basic function everyone can do. If you want to nerf him that badly, I'm willing to satisfy that desire by limiting his semblance instead, shortening its range and adding more caveats to avoid being affected by it.