What is the history of this book? Is it unanimous that it was written by Moshe? If not, by who? If so, how do we have it? Was it passed down orally from generation to generation, until it was eventually written? (I assume it wasn't always in written form)
So the Otzar Midrashim here seems to attribute its authorship to Moshe Rabbeinu:
יש עוד ספר מעין החכמה על התנועות והאותיות ואור הקדמון וכנוי שמות הקדושים ועשר ספירות, ומיוחס למשה רע״ה, ובראש הספר כתוב: זה הספר מעיין החכמה שנתן מיכאל לפאלי ופאלי למרע״ה ומרע״ה גילהו להתחכם בו לדורות וכו׳. בעל סדר הדורות אומר כי מחובר הוא ע״י משה רבינו ע״ה ומעיד שכן כתב הר"ר יוסף גיקטיליא בספרו שערי אורה וכן כתב בעל שפתי ישנים בערכו.
"There is another book - Ma'ayan HaChochma about the phonetics, letters, the primordial light, use of holy names and the ten sefiros. It is attributed to Moshe of blessed memory, and at the beginning of the book it writes: "This book Ma'ayan HaChochma, Michoel gave to Peliy, Peliy to Moshe, and Moshe revealed its wisdom to the generations". The author of Seder HaDoros says that it was authored through Moshe Rabbeinu and was then written by Rav Yosef Gikatilla in his book Sha'arei Orah and similarly by the author of the Sifsei Yeshanim." (My translation)
It is clearly a very deep, mystical book, which from the description, was first handed down by Michoel the angel.
However, I also came across this piece here which is quoting from Reb Tzadok HaCohen of Lublin:
There is also is printed a sefer Mayan Hachochma that is attributed to Moshe rabbeinu which is also quoted in Pardes, Pelach Harimon, in the introduction to Sefer Emek Hamelech and others. It appears to me that it is accepted by the earlier sages that it is a holy book and it seems to me that it was written in the days of the later Geonim or close to that time by one holy person who through his Ruach Hakodesh was able to apprehend what Moshe rabbeinu did receive from Sinai (!) for a Chacham is greater than a prophet as Ramban in Baba Batra 12a writes see there as quoted in Ayin Yaakov.