Why did G-d tell Moshe to stop praying?

Shemos (14:15) Then G-d said to Moses, “Why do you cry out to Me? Tell the Israelites to go forward.

Rashi ( Shemos (14:15)) WHEREFORE CRIEST THOU UNTO ME? — there is no mention that he prayed to God concerning this, but this teaches us that Moses stood in prayer. Whereupon the Holy One, blessed be He, said to him, “It is no time now to pray at length, when Israel is placed in trouble”.

Why did G-d tell Moshe to stop praying?  The Medrash Rabbah says

Shemos Rabbah (21:07) WHEREFORE CRIEST THOU UNTO ME? It is written, Hast thou prepared thy prayer before thy trouble came?  (Job XXXVI, 19).2 Why is this so? R. Eleazar b. Pedath said3: The proverb has it: ' Honour thy physician even before thou hast need of him. R. Simeon b. Lak. ish says: Prepare thy prayer before thy Creator, so that no troubles may befall thee from above. R. Hama b. Hanina said: When Israel departed from Egypt, the angel Samael arose to accuse them. R. Hama added the following explanation in the name of his father: It can be compared to a shepherd who was leading his sheep across a river when a wolf came to attack the sheep. What did the shepherd, who knew well how to deal with such emergencies, do? He took a large he-goat and threw it to the wolf, saying to himself, Let him struggle with this till we cross the river, and then I will return to bring it back. So, when Israel departed from Egypt, the Angel Samael arose to accuse them, pleading before God: ' Lord of the Universe! Till now they have been worshipping idols, and now Thou dividest the sea for them? ' What did God do? He delivered into his hands Job, one of the counsellors of Pharaoh, of whom it is written, And that man was wholehearted and upright (Job I, 1), and said: Behold, he is in thy hands (ib. II, 6). God reckoned: While he is busily occupied with Job, Israel will go through the sea! Afterwards, I will deliver Job; this is why Job said, I was at ease, and He broke me asunder (XVI, 12). ‘I was at ease in the world,’ said Job, but He broke me asunder, He hath taken me by the neck, and dashed me to pieces (ib.), in order to make me the target for His people, for it says, He hath also set me up for His mark (ib.), God delivereth me to the ungodly (ib. 11), He hath delivered me into the hands of Satan; and in order that Israel may not emerge guilty from the trial, He hath delivered me into his hand’; hence, And casteth me into the hands of the wicked (ib.). It was then that God said to Moses: Moses, behold I have given Job over to Satan. It is for you to Speak unto the children of Israel, that they go forward.