When telling Moshe Rabbeinu about the other nine Makkos, Hashem informed him which Makkah was coming, mentioning it clearly in the pasukim. Moshe then conveyed this message to Pharaoh, and he related exactly what Hashem had instructed him to say, without any personal discretion in the wording. Rav Simcha Bunim posits that when Hashem did not mention the Arbeh by name, He was giving Moshe Rabbeinu licence to furnish his own idea of what the next Makkah ought to be. Unlike all the other Makkos, where Hashem legislated in detail precisely what would happen, this Makkah could have consisted of anything Moshe chose.
The other Makkos are not considered a mockery, because they are gezeiros min haShamayim, decrees from Hashem that had to come to fruition. Here, however, Hashem told Moshe that it was up to him. Hashem gave Moshe the opportunity to toy with the Mitzriyim and send them any plague he saw fit.
Moshe was mechadeish the Makkah of Arbeh. He thought of it and decided that this was to be the next plague.
What tool did Moshe employ as he developed this original idea for a Makkah? He used the gezeirah shavah. Noticing the similarity of the words, he was mechadeish – originated his own Torah thought – that just as in Yoel, this Makkah also ought to consist of Locusts.
Pharoah understood that this plague was different. It was coming not only from Hashem, but in this case, Moshe Rabbeinu played a fundamental role in deciding on the Makkah with which he should punish the Mitzriyim. Pharah acknowledged that he sinned to Hashem and to Moshe, because he recognized the unique formulation of this Makkah; this one was also from Moshe.
