I began to think that it was floating sand, but when found it was more like a ghost block made of.
1.place a repeating command block.
2.input command /kill @e[type=falling_block] ,Opening mode is always active.
3.place a sand(or a block affected by gravity), made it falling.
4.now, this block is killed(or still there).but a ghost block place at place where the block.
5.you can put sand above ghost block,sand is floating.

I'm made it in nether, but I think it's going to come into effect in the overworld.
In addition, I found this block can be respawn:use flint and steel to click the place where this block is killed.