I love the sky! I would just suggest adding some cliff edges underneath the parts where the waterfall is bending. Right now the water looks like it's bouncing on it's own since the cliff is smooth all the way down. I'm not sure if I'm making sense, it's kind of hard to explain. I would also taper the waterfalls and make them more narrow on the top than on the bottom.
I agree with Kolby, the sky is super cool and really alien without being cliche :) I'm not 100% percent sure what the structure in the foreground is, so maybe a little clarification there would help. I would also make the background mountain a little lighter to help it feel farther away. Also, all the waterfalls are kind of clumped an equal distant apart from each other, it might be interesting to space them more naturally/randomly :)