Sk8man,
I have noticed the exact same thing with halo's midnight apple -- only worse than you describe. I use a kayfun most of the time, and with my other juices, I can easily go a week and sometimes more before the flavor goes south or I see discoloration of the juice in the tank. With Halo, I get two days at most. And when I open up the kayfun to rebuild, not only are my wick and coil gunked, there is a black deposit running all the way up into the chimney -- and even the inside of the drip tip. This coating wipes off easily enough, but the jet black color I am seeing all the way up in my drip tip has me worried about how much of this stuff I am actually inhaling. I'm not giving it up just yet (just love it too much to leave it), but I have started to try to approximate it via DIY (for those trying the same, experiment with dessert ship and hangsen cigar for the tobacco notes, the apple flavoring of your choice, and a bit of Koolada -- this will get you in the ballpark at least).
My best guess is that Halo must be using some kind of NETs in their tobacco line -- these are known to gunk coils and there have been many spirited debates over whether "coil killing" juices are more hazardous -- or not. Near as I can tell, the answer is: we just don't know. So you'll have to make up your own mind here, but it seems to me that it might at least be worth cleaning / changing your wicks and cleaning out your tanks more frequently with juices like these.
Good luck to you...
Steve.