The walkable grass blends into the background in many areas, and the rock-grass boundary has too much contrast, so it looks like the rocks are the walkable surface, rather than the grass. In addition, the grass has a lot of contrast and no clear "surface".
I think a fix for both problems is to reduce the contrast within the grass, so it reads more like a lightly textured surface, in contrast to the heavily-textured non-walkable areas and to reduce the contrast within the foreground rocks so they don't distract from the grass. You could also hue-shift the grass more towards yellow and the background slightly more towards blue to get more contrast between them that way. Darkening some of the lighter background colours would also help.
Edit: Put together an edit for you to give you some ideas:
I darkened the nearest background layer, and recoloured the bright background grass with some greens from the trees, so the overall contrast in the background is lower.
I made the grass lighter and yellower, and lightened up the mid green in it so that it doesn't contrast as much with the highlights. I also got rid of the dark shadows in the upper blades of the grass, which reduces the contrast within the grass even further.
I reduced the contrast and colour count in the rocks, and lightened up the area where the rocks meet the grass, so that there's no super-dark separation between the grass and rocks, which I feel helps the ground read more as a connected whole instead of looking like the grass floats above the ground.
Edit 2: I think if you're going to have a light foreground and darkish background, the character sprite could stand to be lightened up too.
