Actually, to correct this a bit. This is why the Hebrew language is important here.
Hashem, the Creator of all things did not tell Moses he would be a "God" as in how the word means in the western English language. He told Moses, in Hebrew:
ודבר-הוא לך, אל-העם; והיה הוא יהיה-לך לפה, ואתה תהיה-לו לאלהים
(Translation) He [Aaron] wil speak for you, to the people, and he [Aaron] will be for you for a mouth, and you [Moses] will be to him
Elohim.
The word of focus is (אלהים) "Elohim." The word Elohim, on it own, does not denote a type deity, or something of worship. When used concerning "the" source of creation, Hashem, it talking about the aspects of power, strength, judgement, of Hashem. Elohim in general, comes from the Hebrew root (א-ו-ל) or (א-י-ל) which denotes power, strength, being the head of, etc. The term, in the Hebrew Tanakh, is used to denote also humans who do the will of Hashem or who stand as judges according to the Torah. It would be like saying, power(s), or strong people, things, or what humans perceive to be strong/powerful.
For example, Exodus 21:6 uses the term Elohim to describe Judges when a servant decides to stay in the service of his master.
והגישו אדניו, אל-האלהים
(Translation) And his master will present him/bring him to meet, to the Elohim/Judges.....
So, Pharoah never once thought that Mosese was the Creator of all things. He simply saw Moses as someone who potentially had a certain power/strength which at first Pharoah's servants could imitate but over time it started to become clear that Moses was in charge some powers that came from Hashem. This is why Pharaoh not once regarded Moses as the source of what was happening but instead knew, Hashem was the source it. Some Jewish sources say that initially Pharoah simply thought Moses was able to predict some events based on understanding nature and in a sense forcasting the weather so to speak.
In the video below I explain the root of the word.