This section is pretty much the definition for "rising action". With each paragraph, the tension builds and we climb one step higher towards the peak.
At first, it looks like Bartholomew and the kingdom will escape any real harm, and we're in for a very boring story.
But when the green cloud approaches, it does so with all the speed and aggression of that, ahem, still largely unexplained smoke monster in Lost. (For the love of all that is good, what is it and why are there so many shots from its POV if it's not sentient?)
Anyway, this cloud is definitely not going for any stroll in the park. It comes closer and closer, sweeping past the entire kingdom in no time so it can swirl around the castle turrets. Notice how Seuss describes the kingdom as "sleeping" to emphasize its innocence and vulnerability.
Though the blobs are just the size of grape seeds now, Bartholomew knows they won't be staying that way and he reacts with very rational fears.