Here is what I wrote there, with some addition edits and points:
I don't know about Karaites, but see this post at RationalistJudaism that this was the position of Rambam, as well as that of some Yemenite midrashim:
"Rambam seems to have shared the view found in certain Yemenite Midrashic texts (and see too Ibn Ezra to Shemos 16:13), that manna is essentially a naturally-occurring substance. It was miraculous in it occurring with unnatural properties (according to the Epistle Against Galen) and with constantly fortuitous timing (according to the Guide). (See the extract from Rabbi Nataniel ben Yeshayah, Nûr al-Zalâm, written in 1329, published in Y. Tzvi Langermann, Yemenite Midrash: Philosophical Commentaries on the Torah, pp. 216-217.)"He also links there to a NY Times article about a similar naturally occurring phenomenon:
Also, to Ibn Ezra who rejects someone who suggests that it is natural:
ירקב שם חוי שאמר: כי המן הוא הנקרא בלשון פרסי תרנגבי"ן ובלשון ערבי מ"ן ובלשון לע"ז מנ"א. כי קושיות רבות עמדו עליו. האחת כי איננו יורד היום במדבר סיני כי ההר ידוע ואני ראיתי זה הדומה למן במלכות אלצכי"ר. והוא יורד בניסן ובאייר ולא בחדשים אחרים. ועוד: אם תשימהו לשמש לא ימס. ועוד: כי בלילה לא יבאש. ועוד: כי איננו חזק ואין צורך שידוכנו אדם במדוכה, שיעשה ממנו עוגות. ועוד: כאשר יושם בלשון יומס. ועוד: כי איננו משביע שיוליד דם טוב, רק הוא נכנס ברפואות. ועוד: כי ביום השישי היה יורד משנה. ועוד: כי לא היה יורד בשבת. ועוד: כי ירד לכל מקום שיחנו. ועוד: כי עבר עימהם את הירדן, ולא שבת עד חצי ניסן על דרך הפשט.
Note that Ibn Ezra says ירקב שם חוי, may the name of Chivi rot. This would appear to be a reference to Chivi al Balchi. According to Wikipedia, scholars think he was either a Jew or a Gnostic Christian. Further, even Karaites opposed Chivi as a heretic. To cite Wikipedia:
If some Karaite said it, I assume Ibn Ezra would have known who said it and been able to name him. Rather he gives this Chivi character as saying it, as well as those who follow after him. (See here, in this longer version of Ibn Ezra, where he refers to "Chivi haPoshea vechol hazonin acharav").Hiwi was the author of a work in which he offered two hundred objections to the divine origin of the Bible. Ḥiwi's critical views were widely read, and it is said that his contemporarySaadia Gaon found in Babylonia, in the district of Sura, some school-masters who, in teaching children, used elementary text-books which were based upon Ḥiwi's criticisms. Saadia not only prohibited the use of these books, but combated Ḥiwi's arguments in a work entitled Kitab al-Rudd ala Ḥiwi al-Balkhi. Both Saadia's and Ḥiwi's books are lost.Ḥiwi's book seems to have been one of the most important contributions to skeptical Jewish literature. Only a few of his objections are preserved, in quotations by other authors. In this way it became known that Ḥiwi raised the question why God preferred to live among unclean mankind instead of living among the clean angels (Judah ben Barzillai), and why He required sacrifices and showbread if He did not eat them, and candles when He did not need light. Another objection of his was based on the claim that God broke a promise which He had made under oath. All these objections are preserved in Saadia's Kitab al-Amanat, among twelve other objections of a similar kind, most of which are supposed to have originated with Ḥiwi. They point out several discrepancies in the Scriptures, and infer therefrom a non-divine authorship. Ḥiwi even objected to the teaching of the unity of God, and referred toDeuteronomy xxxii. 9. In this case, as in several others, Saadia combats Ḥiwi without mentioning his name.Some others of Ḥiwi's views are preserved in Ibn Ezra's commentary on the Pentateuch. The passing of the Israelites through the Red Sea Ḥiwi explained by the natural phenomenon of the ebb-tide; and the words "the skin of his [Moses'] face shone" ("ḳaran," literally, "cast horns" or "rays"; Exodus xxxiv. 29) he explained as referring to the dryness of his skin in consequence of long fasting (see Ibn Ezra on the passage in Exodus). Ḥiwi further inquired why manna from heaven no longer descends in the desert of Sinai as it is said to have done in olden times (Ibn Ezra to Ex. xvi. 13).These few instances of Ḥiwi's criticisms are sufficient to show his skeptical and irreverent spirit, the cause of which D. Kaufmann traced back to anti-Jewish polemical Pahlavi literature.Rosenthal (1948) also indicates that all of these Biblical difficulties can be traced to Manichaean dualist views. Karaites and Rabbinites agreed in denouncing Ḥiwi as a heretic. His real surname, "Al-Balkhi," is correctly preserved in one instance only; in all others it is changed into "Al-Kalbi" (אל-כלבי = "the dog-like").
Perhaps Malbim was aware that this was actually a Karaite, or else falsely inferred that the fellow was a Karaite because Ibn Ezra opposed him, and Ibn Ezra often opposed Karaites.
I've looked at a Karaite peirush myself (namely, Aharon ben Yosef) and don't see any mention or endorsement of this idea.
By the way, here is the Malbim on the Torah, but I am not sure where, precisely, he refers to this rejected position.