the horizontal line is happening because each grass cluster is about dead center in the 16x16 tile, if you offset them a bit more (I see that they don't actually line up, but there close enough,) you could fix that up. I am be no means an expert on tiles, but I find it helps if you work details more from corners then the center, just because you end up thinking about them differently and tend to make more variation in the patterns as a result. Obviously, you don't want to start at the first pixel on any given corner, because you wont have room to work, but if you are starting from the center its really easy to just make a similar pattern for all similar things (like grass) and that makes it really hard to break up the repetition. (Point and case, both of your tiles feature a star shape made of four blades of grass pointing in different directions, and although this shape and the blades do read as grass, and look fine on there own, using that same formation over and over again makes it painfully obvious they are being tiled.)