I see two main issues with this piece.
First, you have much lower contrast on the top line of rocks, meaning that it look
"less concrete" than the rest of the wall. If I correctly assume that the platform is for player to run on, it may misguide them is what is solid and what is not.
Second, you don't have a clear lightsource for those rocks below. They look noisily-pillow-shaded. I recommend you try first to shade them with 2 colors only, far away enough to define some shapes, e.g. having the lower half darker part.
http://www.pixeljoint.com/pixelart/37724.htm shows you what you could reach.
http://www.reddit.com/r/PixelArt/comments/179kkw/ccnewbie_rock_tile_set_for_platformer/ on the other hand, suffers from muddy, too organic shapes for being perceived as rocks. Some of yours suffer from this too.
I would recommend
http://www.subtraction.com/2014/06/19/pixel-art-tutorial/ as a reference, too:
HTH.