Not possible at the moment.
You can "fake" it by tying up focus but I don't think that's optimal.
To start you on the right path if you do decide to tackle it, what I mean is that you change your devouring plague button/keybind to point to a macro:
Code:
#showtooltip
/focus [exists,harm,nodead]
/cast [@focus][] Devouring Plague
This will first store your current target in focus and then cast Devouring Plague on it.
You could then make 2 complementary auras:
One that checks for Devouring Plague on your current target and shows the green checkbox.
One that checks for Devouring Plague on focus and shows the blue circle.
Note that if your focus and current target are the same both graphics will show, so you will have to overlay them in a smart way (maybe the checkbox inside the blue circle?)
Like I said this is not exactly optimal as it will tie up your /focus which you might want to use for other things,
but is the only way to get close to that functionality with Power Auras.
To explain Power Auras tracks buffs/debuffs etc by valid unitIDs ("player", "target", "pet", "focus", "raid1", "party2" etc)
and mobs that are not your current target or focus do not have such a persistent unitID.
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