I'll be honest, olive green is one of my least favorite colors as it is. There are a few colors it works well with, but dark grey is not one of them, especially when it's just two random pieces. Having a larger/irregular patch of moss in a different color would make it much more successful. Other than that, I like rocks but whole MOCs out of just rocks are dangerous since I just automatically compare them to some of Professor B's best works, which pretty much nobody can easily surpass. That being said, you have some good techniques and shaping, so good luck.