I wonder if the grass grow and spreads alone?
Not yet. Eventually.
I’m assuming grass will re-grow where it was previously generated considering all the “regeneration” devs keep mentioning. The only problem I could see with it spreading is if it was left to grow by itself eventually there could be massive fields of nothing but grass. Which I would imagine might cause issues when loading in those chunks but I could wrong.
The current worlds are rendered in that way, future worlds will have the new tall grass render method from r100. I don’t believe the plan is for tall grass to spread that way, just grass block to dirt block, like exists in mc. Not confirmed by devs, though. Perhaps they made grass seed so they don’t have to add grass spreading.