Amazing piece! I really like the composition and the little mermaid, it looks great!
Two big highlights for me, where also the colors which I feel that you did an amazing job with, as well as the sky which I really liked how you portrayed!
That being said my main critique for this piece would be, that I would generally try to avoid a lot of single or double pixel placements that don't belong to a specific cluster. This is, because these types of pixels can create a lot of unnecessary noise. And while in some places it works better than others. I would generally try to be a little more careful when placing stray pixels! ( I would try to form distinct pixel clusters)
Regardless, you did a great job with the piece! I really liked the setting and the feel of it, really magical! Well done!