Once it's ready to harvest - ie the fairy has visited and harvested the bubble - it is of no further use.
To get more grass, you'd need to replant a toadstool in the grass pot (the rainbow coloured one).
If it's Jack and the Beanstalk you're doing, you might find this thread helpful:
https://www.fairylandgame.com/fbfairy/forums.php?tid=71350