Really nice piece! I really like the composition here, it looks really good! The colors you choice for it, also look really great and fit the piece really well!
My only critique though would be, that I feel that in a lot of places there is a lot of unnecessary noise. (A lot of dither and a lot of single pixels scattered throughout. Sometimes that works well for the piece (like in the sky) but other times, it makes places feel really noise and clutter (like in the bottom rocks). So my critique would be to try to reduce a lot of the single pixels and try being a lot more specific about the dither placement. (try not to have overlapping dithers or dither cluster)
Other than that I think it is really good piece! I can see you put a lot of effort into it! Well done!