@Jacobite: Thank you. I've put one coat of watered down sepia in the joints and plan on doing several more until it looks like the rust/dirt has built up. Like silt settles in the rocks of a swamp/marsh/river. Was thinking the moss looked a little big as well. Made some the way you suggested for a castle my son painted, will try that on there. Wanted to try actual greens first though to see how it looked. Another thing I tried was taking cotton balls, splitting it up, dunking in a pva glue, water, dark green paint mixture. The pieces came out awesome, but big. Still experimenting with it though to see what effects that I can get outa it. For the vehicles, if I do go with this theme, they will be rusty as well of course, but I can do some larger vines hanging off them or even kind of wrapped around them.
In the deal I got 2x ghost/doomsday arks, 1x night/doom scythe, 1x command/annihilation barge, 40xwarriors, and 20x immortal/deathmarks. So I have a lot to play around with and practice with. Was really torn between this theme and a snow'ish (off white necrons with blue where all the green would normally go). All in all, it's a fun break from painting my DA army.