Learn English – Hyphenation of “prerequisite”

The short answer: it doesn't require a hyphen.

The longer answer:

Generally speaking, you don't need a hyphen in a compound modifier (like "high frequency") if there is no ambiguity without one, according to the Chicago Manual of Style.

If "frequency words" could be considered a compound noun, then we would need a hyphen there; since it cannot, we don't.

A Google Ngram shows that there is no incidence of either "micro-endoscope" or "micro endoscope," but "microendoscope" is used with prevalence. Therefore, it would appear that "microendoscope" is the proper way to write it. The appearance of the other two spellings in the paper to which you refer appear to be typos or errors in editing.

Incidentally, without a hyphen is consistent with how the prefix "micro-" generally appears (e.g., microeconomics, microbiology, microcosm, micromanage, etc.).