Thank you very much for the constructive feedback. I am still learning pixel art (I'm mostly self learned) so there are lots of flaws in my art. But i will do my best to improve and consider your advice next time!

There are some nice elements like overall conception of pose and details like collar bones. But proportions are off, lines of lower half are not smooth, skin too. Size is too big to control it with care.

Here's the best advice I can give:
Take a pen into left hand (it will be a rose) and go to a mirror.
Try to pose like you wanted to draw the mermaid.
Notice if it is a comfortable position.
Notice how the size of knuckle compares to the whole length of arm and how size of head compares to everything else.
Notice how size of upper arm compares to lower arm. Notice that with different angles comparative sizes change too.
Notice how much distinctive shades you see (I usually see two: not shaded and shaded color) and if a line that separates them is fuzzy with waves and bricks in it or not.
Take a quick photo and try to make a sketch from it.
See how it differs from current drawing.

Second best advise I can give is to buy or download a good book on figure drawing (like by Jack Hamm or Andrew Loomis, but NOT, NEVER by Chris Hart) and draw, draw, draw.