You've done a really nice job on this, congrats!

I'm not sure about the tree. It looks like it's on top of the boulder, but the way the picture looks, it's supposed to be on its side, right? If it's hanging off the boulder, I'd suggest you go with a less thriving-looking tree (it looks almost too thriving for growing on a boulder that floats over poisonous fog!) - a windblown, almost naked couple of branches, maybe. If you add a couple of roots that seems to be coming from two boulders close to each other, you could emphasise that they are drifting apart, but has been closer once.