Glyph origins of the character 微
《說文・彳部》

　
隱行也。从「彳」，「」聲。
「微」 (Baxter-Sagart OC: /*məj/), to travel [somewhere] under stealth. From semantic 「彳」 (to walk) and phonetic 「」 (/*məj/).


Strictly speaking, 「微」 is just a complexified character from 「」. Characters naturally become more complex over time for disambiguation purposes, as simpler characters tend to be used and reused for other senses or as rebus characters/phonetic loans, causing them to be overloaded with definitions and more easily subjected to shape distortions, and thus increasingly difficult to comprehend.
Since 「」 is the original character for the meaning small, slight, then in terms of glyph origins, 「微」 is built from semantic 「彳」 and doubly semantic and phonetic 「」 . 「彳」 is simply a semantic indicator appended on to 「微」 to extend the meaning small, slight to travelling under stealth.
So, all this begs the question - what exactly is 「」 supposed to be？

Glyph origins of 「」
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「」 is constructed from semantic 「髟」 (picture of a person with long hair) and semantic 「攵・攴」 (picture of a hand holding a hitting implement &gt; to strike, to hit), inferring the meaning delicate, slight through the action of cutting off a full growth of hair into a more elegant and delicate state.
Shuōwén's

《說文・人部》

　
妙也。从「人」，从「攴」。「豈」省聲。
(/*məj/) [means] delicate. From semantic 「人」 (person), semantic 「攴」 (to strike), and reduced phonetic 「豈」 (/*C.qʰˤəjʔ/).


is probably a misunderstanding. 「豈」 may have originated from an alteration of 「壴」 (picture of a drum) into 「豈」 (beating of victory drums &gt; triumphant music, now written as 「凱」), and Shuōwén has confused the top part of 「豈」 with the top-[left] part of 「髟」.

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For reference, shape derivation of 「豈」. Shapes 1-4 refer to 「壴」. Shape 5 is 「敳」, with the left-hand component as 「壴」 but the top-left part written as a rotated 「山」. This sets the scene for the confusion of 「豈」 and 「」 in Shuōwén. Forms 6-9 refer to 「豈」, which all inherit the (rotated) 「山」 shape.


References:

季旭昇《說文新證》
李學勤《字源》
漢語多功能字庫
小學堂字形演變
國學大師

郭沫若《甲骨文合集》
中國社會科學院考古研究所《殷周金文集成》