וַיִּצְעַק מֹשֶׁה אֶל ה' עַל דְּבַר הַצְפַרְדְּעִים אֲשֶׁר שָׂם לְפַרְעֹה (ח, ח)
And Moshe cried out to Hashem about the matter of the frogs that were placed on Pharaoh. (8:8)
Sefer Tzemach Dovid asks why the word ‘vayitzak’ (and he cried out) is only used in regards to this makkah. Secondly, he notes that the word ‘devar’ (the matter of) seems unnecessary.
He answers by quoting the Medrash (Yalkut Shimoni Tehillim, Remez 889) that says that frogs sing a lot of shira to Hashem – even more than Dovid Hamelech. The Gemara says (Pesachim 53B) that the frogs sacrificed their lives to perform their mission by jumping into the fiery ovens. Chananya, Mishoel and Azariah learned from them that they too must sacrifice their lives for Hashem. It is certain that the frogs sang shira when they did this great mitzvah. Therefore, Moshe was reluctant to bring an end to this makkah and stop the shira. When it came time that he had to end it, he davened with ‘tze’aka’, which means a prayer said in the heart – not out loud (see Zohar Hakadosh, Shemos 20A). He did not want to openly daven to stop the shira, so he davened silently, in his heart.
The reason he did, in fact, daven to end the makkah was ‘because of the matter of the frogs that were placed in Pharaoh.’ The frogs that were inside the stomachs of the Egyptians were also saying shira. But because they were in an unclean place, it was not respectful for them to be praising Hashem. Therefore, Moshe had a reason to stop the makkah.