tile.movingBlock.name drops while digging moving cobblestone

I made a simple cobblestone generator, just produce cobblestone with lava and water streams and then push it out of the way with a piston. While I collecting them with a iron pickaxe, a block as shown in the video attached drops. It‘s name was "tile.movingBlock.name" in the inventory.

PS. It happens on my realm with a long delay.