Pistons can't push slime blocks similar to bedrock and obsidian, appears that this doesn't affect sticky pistons in any way. I showed the debug screen to show the lever is on.
MC-73548
Piston can't push slime block
Resolved
Invalid
0
Slime blocks will attempt to push any blocks connected to it (in your case, the grass), but if it fails to move a movable block, it will not move.